Carbon-polymer electrochemical systems and methods of fabricating them using layer-by-layer technology

ABSTRACT

One aspect of the invention provides ion-exchange and gas-diffusion membranes, fabricated by a layer-by-layer approach, for use, e.g., in electrochemical cells; a process for making membrane electrode assemblies fabricated using porous frameworks, LBL composite membranes and LBL carbon-Polymer electrodes; and the application of the membrane and electrode assemblies to a variety of devices, both electrochemical and otherwise.

GOVERNMENT SUPPORT

This invention was made with support from the National ScienceFoundation (grant number CTS-0136029); therefore, the government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

Electrochemical cells in which a chemical reaction is forced by addingelectrical energy are called electrolytic cells. Central to theoperation of any cell is the occurrence of oxidation and reductionreactions which produce or consume electrons. These reactions take placeat electrode/solution interfaces, where the electrodes must be goodelectronic conductors. In operation, a cell is connected to an externalload or to an external voltage source, and electric charge istransferred by electrons between the anode and the cathode through theexternal circuit. To complete the electric circuit through the cell anadditional mechanism must exist for internal charge transfer. This isprovided by one or more electrolytes, which support charge transfer byionic conduction. Electrolytes must be poor electronic conductors toprevent internal short circuiting of the cell.

The simplest electrochemical cell consists of at least two electrodesand one or more electrolytes. The electrode at which the electronproducing oxidation reaction occurs is the anode. The electrode at whichan electron consuming reduction reaction occurs is called the cathode.The direction of the electron flow in the external circuit is alwaysfrom anode to cathode.

A typical electrochemical cell will have a positively charged anode anda negatively charged cathode. The anode and cathode are typicallysubmerged in a liquid electrolytic solution which may be comprised ofwater and certain salts, acids or base materials. Generally speaking,the anode and cathode are made of substrate materials such as titanium,graphite, or the likes coated with a catalyst such as lead dioxide orother known materials. The selection of a substrate and catalyst isdetermined by the particular electrode reactions which are to beoptimized in a given situation.

The cathode and anode are positioned within the electrolytic cell withelectrical leads leading to the exterior. The cell may be provided withappropriate plumbing and external structures to permit circulation ofthe electrolyte to a separate heat exchanger. Suitable inlet and outletpassages may also be provided in the cell head space to permit thewithdrawal of the gases evolved from the cathode (if gases are to beevolved) and from the anode.

In order to maintain or reduce the temperature of the cell electrodes,heat exchange passages may be provided within the electrode structures.These coolant passages are connected to external sources of coolantliquid which can be circulated through the electrodes during theelectrolysis process in order to maintain or reduce their temperatures.

In order to drive the electrolysis reactions it is necessary to applyelectric power to the cell electrodes. The electrodes are connectedthrough the electrical leads to an external source of electric powerwith the polarity being selected to induce the electrolyte anion flow tothe anode and the cation flow to the cathode.

Layer-by-Layer (LBL) Technique

Organic thin films continue to attract great interest in the materialsscience community due to their ease of processing, ease offunctionalization, light weight and flexibility. Significant progresshas been achieved in the past 10-20 years, presenting the possibility ofmolecular-level control in molecular and macromolecular composite films.The ionic, layer-by-layer assembly technique, introduced by Decher in1991, is among the most exciting recent developments in this area.Makromol. Chem., Macromol. Symp. 1991, 46, 321; Ber. Bunsenges. Phys.Chem. 1991, 95, 1430; Thin Solid Films 1992, 210/211, 831. Thisapproach, which utilizes electrostatic interactions between oppositelycharged polyion species to create alternating layers of sequentiallyadsorbed polyions, provides a simple and elegant means of depositinglayer-by-layer sub-nanometer-thick polymer films onto a surface usingaqueous solutions. Crystallography Reports 1994, 39, 628; Macromol.1995, 28, 7107; Langmuir 1997, 13, 2171. This layer-by-layer depositionprocess provides a means to create polycation-polyanion polyelectrolytemultilayers one molecular layer at a time, thereby allowing anunprecedented level of control over the composition and surfacefunctionality of these interesting materials. Typically, alternatelayers of positively and negatively charged polymers are sequentiallyadsorbed onto a substrate from dilute solution to build upinterpenetrated multilayer structures. Most studies have focused onpolyelectrolytes in their fully charged state, such as strongpolyelectrolyte poly(styrene sulfonate) (SPS). However, we havediscovered unique properties when at least one alternating layer in thepolyelectrolyte multilayer is a weak polyelectrolyte where the chargedensity along the chain can be readily controlled by adjusting the pHvalues of the polyelectrolyte solution. Thin Solid Films 1992, 210, 831.

More recently, applications have been extended to electroluminescentLEDs, conducting polymer composites, and as the assembly of proteins andmetal-nanoparticle systems. Adv. Mater. 1995, 7, 395; Adv. Mat. 1998,10, 1452; Thin Solid Films 1994, 244, 985; Thin Solid Films 1994, 244,806; J. Am. Chem. Soc. 1995, 117, 6117. The electrostatic LBL techniquehas been extended to include many charged systems other than polymersand even other complexation mechanisms, such as hydrogen bonding. Chem.Lett. 1997, 125; Macromol. 1997, 30, 2717.

Solid Polymer Electrolytes (SPEs)

As mentioned above, all electrochemical systems consist of electrodesseparated by an electrolyte for ion conduction and a load for electronicconduction, as electricity can be generated or fed into the system.Early electrochemistry relied exclusively on liquid electrolytes, butrecent applications are more demanding. Solid polymer electrolytes(SPEs) have replaced liquid electrolytes in many high-performanceapplications, such as batteries, fuel cells, sensors, and electrochromicdevices. Compared to liquid electrolytes, SPEs feature easierprocessing, enhanced chemical compatibility, and better mechanicalproperties with only a modest decrease in conductivity.

A major advantage gained from forming SPEs by the LBL technique is theintroduction of a large number of variables that modify the electrolyteor the electrodes depending on the user's application. Other advantagesinclude the utilization of cheap nontoxic polyelectrolyte materials, aneconomic and simple fabrication process, and miniaturization of theelectrochemical components. For example, a composite membrane made byLBL deposition of a poly(+)/poly(−) couple on a porous framework is morethan ten times cheaper than any common commercialproton-exchange-membrane (PEM), yet it can deliver more than half thepower. In addition, a stainless steel composite electrode made by LBLdeposition of a colloid of platinum/carbon catalyst with apoly(−)/poly(+) stabilizers acted similar to a pure platinum electrodeby furnishing the same open-circuit potential yet it is a thousand timescheaper and, unlike solid platinum, allows the conduction of ions.

Fabrication of Fuel Cells Via LBL

A fuel cell is a type of electrical energy generating device. There areseveral types of fuel cells such as acid fuel cells, molten carbonatefuel cells, solid polymer electrolyte fuel cells and solid oxide fuelcells. A fuel cell is an apparatus for continually producing electriccurrent by electrochemical reaction of a fuel with an oxidizing agent.More specifically, a fuel cell is a galvanic energy conversion devicethat chemically converts a fuel such as hydrogen or a hydrocarbon and anoxidant that catalytically react at electrodes to produce a DCelectrical output. In one type of fuel cell, the cathode materialdefines passageways for the oxidant and the anode material definespassageways for fuel. An electrolyte separates the cathode material fromthe anode material. The fuel and oxidant, typically as gases, arecontinuously passed through the cell passageways for reaction. Theessential difference between a fuel cell and a battery is that there isa continuous supply of fuel and oxidant from outside the fuel cell. Fuelcells produce voltage outputs that are less than ideal and decrease withincreasing load (current density). Such decreased output is in part dueto the ohmic losses within the fuel cell, including electronicimpedances through the electrodes, contacts and current collectors. Aneed therefore exists for fuel cells which have reduced ohmic losses.

Recently, industrial nations have revived the usage of alternativeenergy sources to address their energy problems. At the forefront ofalternative energy technologies are fuel cells which consume hydrogen ormethanol, rather than crude oil, to generate electricity. Larminie, J.;Dicks, A. Wiley, New York 2000. Although fuel cell technologies arerelatively well known, there is a strong need for more portable,lightweight and low-cost fuel cell devices for portable devices,micropower applications, and new applications requiring embedded powerin textiles, paper, plastics and other thin film geometries. Using thelayer-by-layer (LBL) self-assembly a new generation of fuel cells can beenvisioned. Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992,210/211, 831-835; Arys, X.; Jonas, A. M.; Laschewsky, A.; R., L. 2000,505-564. Micro fuel cells assembled using the LBL technology, arefundamentally different from those described in the literature. Mostauthors use expensive lithographic and sputtering techniques tofabricate a large number of microelectrodes on a flat substrate and useconventional PEMs as separators. However, micropatterned LBL fuel cellswould provide access to low cost, readily available, and easilymass-produced micropower devices analogous to, but much cheaper than,the traditional microelectronic processes. Such systems might includethe use of an ultrathin perm-selective membrane on a porous, ionicallytransmissive support. A major advantage of PEMs over classical membranesis that extremely thin films can effectively reduce the flow of specificgases, while maintaining a high flux of others. Krasemann, L.; Tieke, B.Journal of Membrane Science 1998, 150, 23-30; Krasemann, L.; Tieke, B.Material Science and Engineering 1999, 819, 513-519; Krasemann, L.;Tieke, B. Mat. Sc. Eng. C-Bio S89 1999, 513-518; Krasemann, L.; Tieke,B. Langmuir 2000, 16, 287; Krasemann, L.; Tieke, B. Chem. Eng. Tech.2000, 2, 211-213. With a typical thickness per layer pair of about 1.0to about 100 nm, it is possible to engineer a broad range of systemswhich will act as effective components in proton exchange membranes.Krasemann, L.; Tieke, B. Journal of Membrane Science 1998, 150, 23-30;Levasalmi, J. M.; McCarthy, T. J. Macromolecules 1997, 30, 1752.

The core of a fuel cell is the membrane-electrode assembly (MEA). TheMEA of a fuel cell is usually fabricated by sandwiching aproton-exchange membrane (PEM) between two gas diffusion C/Ptelectrodes. Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield,S.; Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301. Themost commonly used PEMs are the perfluorosulfonated membranes (e.g.,Nafion®) which are comprised of a PTFE crosslinked hydrophobic backboneimpregnated with hydrophilic sulfonic acid sites needed for protonmobility. Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield, S.;Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Mehta, V.;Cooper, J. S. J. Power Sources 2003, 114, 32-53. Other types ofmembranes used as PEMs are the hydrocarbon polymer, non-fluorinated, andpolymer-inorganic composite membranes that, in general, are lessexpensive and recyclable. Glipa, X.; Hograth, M. Dept. of Trade andIndustry (UK) homepage 2001; Panero, S.; Ciuffa, F.; D'Epifano, A.;Scrsati, B. Electrochim. Acta 2003, 48, 2009-2014; Rikukawa, M.; Sanui,K. Prog. Polym. Sci. 2000, 25, 1463-1502. Some polymers such as thepolyphosphazenes, the polybenzimidazoles (PBI) and zirconia-polymer gelsexhibit an equal or better performance than the conventionalperfluorinated membranes, especially for water retention at hightemperature. Qunhui, G.; Pintauro, P. N.; Tang, H.; O'Connor, S. J. Mem.Sci. 1999, 154, 175-181; Glipa, X.; Bonnet, B.; Mula, B.; Jones, D. J.;Rozier, J. J. Mater. Chem. 1999, 9, 3045-3049; Alberti, G.; Casciola, M.Solid State Ionics 2003, 145, 3-16. However, it should be noted that thepolyphosphazenes and the zirconia-polymer gels are not commerciallyavailable and the PBIs are relatively expensive. An emerging membranetechnology based on the layer-by-layer deposition of polyelectrolytesmultilayer films on solid substrates or detachable films might beharnessed to perform like a classical PEM. Decher, G.; Hong, J. D.;Schmitt, J. Thin Solid Films 1992, 210/211, 831-835; Dubas, S. T.;Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368-5369;Vazquez, E.; Dewitt, D. M.; Lynn, D. M.; Hammond, P. T. J. Am. Chem.Soc. 2003, 125, 11452; Arys, X.; Jonas, A. M.; Laschewsky, A.; R., L.2000, 505-564.

Because the LBL films can be tailored to deposit any polyelectrolyte(PE) couple to any desired thickness, ranging from a few angstroms to afew microns, they are much less expensive technology than conventionalmembranes. Ion permeability and ion conductivity in LBL films have beenextensively studied and characterized. Krasemann, L.; Tieke, B. Langmuir2000, 16, 287; Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17,1184-1192; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125,4627-4636; Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem.Soc. 1999, 121, 1978; DeLongchamp, D. M.; Hammond, P. T. Chem. Mater.2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am.Chem. Soc. S22:136-PMSE, Part 2 2001. The diffusion coefficient of ionsof conventional polymer multilayers is a few orders of magnitude lowerthan the classical ion exchanger membranes hence their proton conductionis lower. However, a range of multilayer systems which incorporatehydrophilic polymers using electrostatic and hydrogen bondingmechanisms, and have shown increases in ionic conductivity of 3 or 4orders of magnitude. DeLongchamp, D. M.; Hammond, P. T. Chem. Mater.2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am.Chem. Soc. S22:136-PMSE, Part 2 2001; Tokuhisa, H.; Hammond, P. T. Adv.Funct. Mater. 2003, 13, 831-839. These differences are further enhancedby the fact that ultra thin films can be formed using the LBL technique,making the final conductance closer to that required for powerapplications. One can tune the thickness and permeability, as well asthe composition, of these films through choice of polyelectrolytes andadsorption conditions. For example, using strong polyelectrolytes withhydrocarbon backbones yields LBL films that tend to be either stronglyor moderately hydrophobic, thus discouraging proton exchange. On theother hand, LBL films assembled using weak electrostatic and secondaryinteractions (i.e. long-range hydrogen bonding or dipole-dipole),particularly those with hydrophilic backbones, support proton-exchange.

The advantages gained using polyelectrolytes to synthesize the LBL PEMmembrane should apply to the synthesis of LBL electrodes. On top of fastion conduction LBL electrodes demand high electronic conduction,strongly hydrophobic to expel water, stable to chemical and mechanicaldegradation, assessable to control loading of catalysts, intimatelyadhere to the PEM and the GDL to ensure proper passage of the ions,capable of producing open-circuit-potentials similar to a pure metal.Larminie, J.; Dicks, A. Wiley, New York 2000; Gottesfield, S.;Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Glipa, X.;Hograth, M. Dept. of Trade and Industry (UK) homepage 2001. Conductingpolymers were successfully used to assemble LBL electronicallyconducting films. Rubner, M. F.; Stockton, W. B. Macromolecules 1997,30, 2717-2725; Rubner, M. F.; Fou, A. C. Macromolecules 1995, 21, 7115.;Rubner, M. F.; Cheung, J. H.; Fou, A. F. Thin Solid Films 1994, 244,985; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc.S22:136-PMSE, Part 2 2001;. DeLongchamp, D. M. PhD Thesis, MassachusettsInstitute of Technology, MA 2003. Unfortunately, LBL conducting polymerfilms are weak ionic conductors, not stable and degrade in a severelectrochemical environment. A more resilient combination is apolyelectrolyte-colloid such that the colloid is electronicallyconducting and ready to assemble. Many colloids can assemble withpolyelectrlytes but the LBL films are not conducting. Kotov, N. A.;Dékany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065.; Mallouk, T.E.; Feldheim, D. L.; Crabar, K. C.; Natan, M. J. J. Am. Chem. Soc. 1996,1181, 7640-7641.; Grabar, K. C.; Natan, M. J.; Freeman, R. G.; Hommer,M. B. Anal. Chem. 1995, 67, 735-743.; Hammond, P. T.; Rubner, M. F.;Zheng, H. P.; Lee, I. Adv. Mater. 2002, 14, 569-572. Only one originalapproach used exfoliated graphite oxide that is not conducting to makeLBL films because graphite cannot be dispersed in water and it formsmicrometer-sized irregular aggregates in organic solvents. The GO can beconverted to graphite under sever reduction conditions with H₂ gas.Fendler, J. H.; Cassagneau, T. Adv. Mater. 1998, 10, 877-881.; Kotov, N.A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637. Our methoddirectly employs polyelectrolyte graphite mixtures to assemble LBLelectrodes without having to convert the graphite powder to exfoliatedGO and then back to graphite where in both processes expensive and severchemical and thermal conditions applies. The LBL polyelectrolyte-Carbonelectrodes [LPCE] achieved most of the requirements stated above, thusproviding a cheaper and practical way of making electrochemicalelectrodes.

SUMMARY OF THE INVENTION

The invention provides improved ion exchange and gas diffusion membranesfor use in electrochemical cells, a process for making porousframeworks, membrane and electrode assemblies fabricated using porousframeworks, and the application of the membrane and electrode assembliesto a variety of devices, both electrochemical and otherwise.

BREIF DESCRIPTION OF THE FIGURES

FIG. 1 depicts SEM pictures of LBL films composed of (a) PANI/PAMPS [10kV, M=11000] covering the surface of a Nucleopore membrane with thepores appearing in the crevice; and (b & c) PEO/PAA [15 kV, M=8000,P=1.9 Torr] with the Nucleopore clearly sandwiched between the LBL films(a cross sectional view). Also shown a simplified sketch of the profileof the “composite membrane”.

FIG. 2 depicts (a) normalized drift velocity against the fraction ofsites in a porous (⋄) and a nonporous (▪) matrix. A 2D coordinate spaceof 400×200 points, randomly distributed hopping sites [5000], a set of[50 or a 100] conducting points making up a wave packet; (b) asymmetricmode of hopping of a conducting point; and (c) normalized drift velocityagainst the fraction of sites in a pore (i) Saturation of sitesrepresenting aqueous phase (⋄), (ii) 60% population of sites (♦), (iii)20% population of sites (Δ), (iv) 10% population of sites (▴). [%population is out of 5000 hopping sites].

FIG. 3 depicts (a) AC impedance of a PDAC/PAMPS fuel cell running underdry conditions (RH<30%, □) while the humidity was ramped up to RH=50 to70% (♦), and then to operating conditions Solid line (RH˜90%); and (b)as the fuel cell was further purged to saturation the high frequencysemi-circle referring to the pore's impedance disappeared and theoverall impedance Solid line sharply decreased. Frequency=1 to 2×10⁷ Hz,Applied potential=10 mV. T=22-24° C. Other PE couples exhibited similarbehavior.

FIG. 4 depicts (a) A system composed of two Randel's cells connected inseries showing a Warberg constant-phase element (b) AC impedance plotssimulated using the circuit in part (a) at (i) RH<30% (□), R_(m)=390Ω,R_(p)=590Ω, C_(m)=4×10⁻⁴F, C_(p)=10⁻⁵F, C_(W)=0.012F, φ=0.8; (ii) RH=50to 70% (♦), R_(m)=220Ω, R_(p)=270Ω, C_(m=)2×10⁻⁴F, C_(p)=2×10⁻⁶F,C_(W)=0.012F, φ=0.8; (iii) RH˜90% Solid line, R_(m)=110Ω, R_(p)=141Ω,C_(m)=2×10⁻⁴F, C_(p)=2×10⁻⁶F, C_(W)=0.012F, φ=0.8; (c) A fully purged“composite membrane” exhibits a sharp drop in pore resistance and adecrease in the overall impedance, Solid line, R_(m)=22Ω, R_(p)=3Ω,C_(m)=2×10⁻⁴F, C_(p)=2×10⁻⁶F, C_(W)=0.012F, φ=0.8.

FIG. 5 depicts ((a) Potential (V) vs Current density (mA cm⁻²) forLPEI/PAMPS ▪, LPEI/PSS □, PEO/PMAA ▴; (b) Corresponding power densityplots for the same set of membranes. Conditions: LPEI/PAMPS, P_(air)=2.0psi, P_(H2)=0.5 psi, RH=50-65%. LPEI/PSS P_(air)=3.0 psi, P_(H2)=0.5psi, RH=65-75%. PEO/PMAA P_(air)=2.0 Psi, P_(H2)=0.5 psi, RH=55-88%.Temperature=22.5-24° C. Area of the electrode is 0.5 cm⁻². Solid linesare guide to the eye

FIG. 6 depicts (a) Potential (V) vs Current density (mA cm⁻²) forLPEI/PAA Δ, PDAC/PAMPS ▴; PEO/PAA ◯; (b) Corresponding power densityplot for the same set of membranes. Conditions: LPEI/PAA P_(air)=2.0psi, P_(H2)=0.5 psi, RH=55-75%. PDAC/PAMPS P_(air)=2.0 psi, P_(H2)=0.5psi, RH=51-60%. PEO/PAA P_(air)=2.0 psi, P_(H2)=0.5 psi, RH=50-60%.Temperature=22.5-24° C. Area of the electrode is 0.5 cm⁻². Solid linesare guide to the eye.

FIG. 7 depicts a soft fuel cell assembled on an insulating customizedporous support.

FIG. 8 depicts an electrode-catalyst layer atop of the GDL.

FIG. 9 depicts an electrode-catalyst layer within the GDL and the LBLmembrane contains a colloidal porous framework.

FIG. 10 is a schematic of a Soft Carbon-Polymer Galvanic cell.

FIG. 11 depicts a miniature micro fuel-cell.

FIG. 12 is a sketch of a small part of a catalytic converters.

FIG. 13 depicts a thick, highly porous LPCE built on a porous membrane.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described more fully with reference to theaccompanying examples, in which certain preferred embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

I. Process for Making Ion-Exchange Membranes (IEMs)

Polyelctrolytes Used in IEM Fabrication. The polyelectrolytes utilizedwere taken from the group consiting of poly(styrene sulfonic acid,sodium salt) [PSS; MW=70,000]; poly(diallyldimethyl ammonium chloride)[PDAC; MW=240,000]; linear poly(ethyleneimine) [LPEI; MW=25,000];poly(acrylic acid) [PAA; MW=90,000]; poly(styrenesulfonic acid-maleicacid, sodium salt) [PSSM3:1; MW=20,000]; poly(ethylene oxide) [PEO;MW=4,000,000]; poly(methylacrylic acid) [PMA; MW=100,000];poly(acrylic-co-acrylamide acid, sodium salt) [PAAcoAAm, MW=10,000,00040% carboxy]; poly(2-acrylamido-2-methyl-1-propane sulfonic acid)[PAMPS=2,000,000]; poly(dimethylamine-co-epichlorohydrin) [PDME];polyaniline (Emarlidine base) [PANI, MW=100,000].

Choice of Polyelectrolytes. Ion exchange membranes (IEM), particularlythe proton-exchange membrane (PEM) of fuel cells, are used in solidstate electrochemical systems to replace the aqueous electrolyte.Various types of IEMs, of acceptable ionic conductivity, can beassembled using LBL technique. At least two types of water-solublemacromolecules capable of electrostatic or secondary interactions shouldbe utilized, and one of them should promote ionic conductivity. Theconcentrations of the macromolecular solutions can range from about 1.0mM to about 20.0 mM, regardless of the nature of the polyelectrolyte.The pH of assembly is relevant only in the case of LBL films whichincorporate weak polyelectrolytes (i.e. polyelectrolytes where thecharge density along the chain can be readily controlled by adjustingthe pH values of the polyelectrolyte solution). For example, weakpolyelectrolytes such as LPEI/PAA at pH=4, PEO/PAA at pH=2.5 or atpH=2.0, PAAm/PAA at pH=2.8, PAAm/PAA-co-PAAm at pH=2.0, andPDAC/PAA-co-PAAm at a pH>5 form IEMs of acceptable ionic conductivity.In contrast, the LBL assembly of strong polyelectrolyte couples is pHindependent; however, in these cases the thickness of the film dependson the concentration of the dosing salt (i.e. 10.0 mM PDAC (0.5MNaCl)/5.0 mM PAMPS (0.5M NaCl) at any pH).

Water solubility of all the films components is not a limitation whenforming IEMs by the LBL technique. For example, ion conducting colloids(e.g., cationic or anionic latexes, zeolites, and zirconia salts) canalso incorporated into LBL films to form IEMs. Colloids that possessstrong functional groups, such as sulfonates, phosphates, and quaternaryammonium groups are pH independent, while those with weak functionalgroups, such as carboxylates, primary, and secondary amine groups, arepH dependent and can only assemble LBL films of the instant invention ata pH range of about 2 to about 4. In addition, cationic and anionicdendrimers, which fall under the same category as the ion conductingcolloids, can also be assembled into films under similar LBL conditionsto form IEMs.

Choice of Substrates for LBL deposition. It will be appreciated thatmaterials with an inherently charged surface are particularly attractivesubstrates for LBL assembly of an inventive thin film. Alternatively, arange of methods are known in the art that can be used to charge thesurface of a substrate, including but not limited to plasma processing,corona processing, flame processing, and chemical processing (e.g.,etching, micro-contact printing, and chemical modification). Forexample, plastics can be used as substrates, particularly if they havebeen chemically modified to present polar or charged functional groupson the surface.

In one approach to IEM fabrication, the LBL film can be depositeddirectly onto a hydrophilic porous framework. Porous polymer membranesinclude all types of organic and inorganic, nano or micro-pore filtermembranes which can be made hydrophilic, as mentioned above, by plasmaetching with an acidified dichromate solution, a H₂O₂/H₂SO₄ solution ora H₂O₂/NH₃ solution. After etching, the porous substrate can be coatedon both sides by an LBL multilayer film to form a composite membranewhich can act as an IEM.

Interestingly, in some cases the LBL IEM can be directly affixed to anelectrode to form a membrane-electrode assembly (MEA). It is importantto note that depositing an LBL film on the surface of an electroderequires a membrane thickness that is at least two times thicker thanthe rough surface of the electrode; the requirement is to preventshort-circuiting of the cell after the electrodes are pressed together.A more detailed discussion of MEAs is found in a subsequent section.

Certain LBL membranes themselves can be made hydrophilic (i.e. convertedto a porous structure suitable for LBL deposition) simply by changingthe pH or salt concentration (e.g., assemble 10.0 mM LPEI/PSSM at pH=4then change the pH to <2.0, similarly this can be done with LPEI/PAA,PAH/PAA) thus forming a framework which can be coated on both sides by asecond LBL multilayer film to form a composite membrane which can act asan IEM.

Similarly, if an LBL film is made from polyelectrolytes and micro ornano latex spheres, zeolites, platelets, or other colloidal particulates(e.g., 10.0 mM PDAC/2% sulfonate latex suspension), it can by fabricatedin such a way that it posses crevice structures, thereby generating aporous framework which can be coated on both sides by a second LBLmultilayer film to form a composite membrane which can act as an IEM.

Methods of Assembly of LBL Films. In certain embodiments, the LBLassembly of inventive films may involve a series of dip coating steps inwhich the substrate is dipped in alternating polycationic andpolyanionic solutions. Additionally or alternatively, it will beappreciated that deposition of alternating polycationic and polyanioniclayers may also be achieved by spray coating, brush coating, rollcoating, spin casting, or combinations thereof.

Examples of Post Fabrication Modifications. The synthesis of the IEM LBLmultilayer film is a fully controlled process. For example, an LBL filmmade from one type of macromolecules can be capped with a thinner LBLfilm of another type of macromolecules (e.g to repel water); or an LBLfilm can be chemically, thermally or photochemically treated to inducecross-linkages, thus enhancing its chemical and mechanical stability; oran LBL film can be tailored to the desired thickness (i.e. up to severalmicrometers) and to the desired size (i.e. from meters down to a fewmicrons).

Properties of Composite Membrane. Since at least two water-solublemacromolecules are needed to make up an LBL film, it follows that acomposite membrane will possess different chemical and physicalproperties depending on the chemical structure of the constituentmacromolecules. Some examples of this phenomenon include: an LBL filmcontaining zeolite clusters blocks methanol permeation, while an LBLfilm containing only polyelectrolytes cannot; an LBL film capped withinorganic colloids or latexes resists peroxide degradation; an LBL filmcapped with a hydrophobic polyelectrolyte repels water at the cathode;and most importantly, in the case of the instant invention, theobservation that hydrophobic polyelectrolytes (e.g., PDAC/PSS) exhibit alower ionic conduction than hydrophilic polyelectrolytes (e.g.,LPEI/PAA).

Interestingly, an LBL membrane of the present invention behaves as asponge which can absorbs ions allowing the enhancement of the ionicconductivity of the film. Depending on the materials used to form thecomposite membrane, either the matrix or the pores can be embedded withchemicals to enhance the ionic conductivity of the membrane. Chemicalswhich can be embeded can be selected from the following: hydrogels(e.g., PEG, OEGDA, PAAm, PVA, PVP); polyions (e.g., polyphosphates,Nafion®); micron or nano-size colloids, platelets, and zeolites; alltypes of proton sponges; organic molecules (e.g., ethylene glycol andglycerol) that are known to increase proton conductivity; and all typesof acids and salts of an organic and inorganic nature.

In an example of this process, a composite membrane (or the soft MEA)can be soaked in about 40.0 mM of the dosing solution (range about 10.0to about 50.0 mM; 1% to 5% for some polymer solutions) for about 30minutes (range 5.0 minutes to 24 hour; depending on the size of themolecule) and an increase in ionic conductivity will be observed. Forexample, a soft MEA [PDAC/PAMPS membrane-electrode] soaked in 2.5%Nafion117 solution showed a ten fold increase in its ionic conductivity.

IIa. Membrane-Electrode Assemblies (MEAs) to Form LPCEs

Overview of the Fabrication of LBL polyelectrolyte-carbon electrodes(LPCEs). LPCEs are formed by standard LBL techniques usingpolyelectrolyte-carbon-catalyst colloidal suspensions. LPCEs, along withIEM can be combined to form soft membrane-electrode assemblies (MEAs).There are two conventional methods that electrochemical and fuel celltechnologists use to fabricate a membrane-electrode assembly (MEA). Onemethod is direct application. Therein one deposits the LPCE directly onany classical IEM (e.g., PSS gel, Nafion®) or an IEM consisting of anLBL composite membrane (as described above in Section I). There is alsoan indirect application approach. Therein one deposits the LPCE directlyon any gas diffusion substrate that is electronically conductive (e.g.,carbon cloth, porous stainless steel, porous silicon, porous titaniumalloys, etc.) to form the catalyst layer.

Polyelctrolytes Used in LPCE Fabrication. The polyelectrolytes utilizedwere taken from the group consisting of poly(styrene sulfonic acid,sodium salt) [PSS; MW=70,000]; poly(diallyldimethyl ammonium chloride)[PDAC; MW=240,000]; linear poly(ethyleneimine) [LPEI; MW=25,000];poly(acrylic acid) [PAA; MW=90,000]; poly(styrenesulfonic acid-maleicacid, sodium salt) [PSSM3:1; MW=20,000]; poly(ethylene oxide) [PEO;MW=4,000,000]; poly(methylacrylic acid) [PMA; MW=100,000];poly(acrylic-co-acrylamide acid, sodium salt) [PAAcoAAm, MW=10,000,00040% carboxy]; poly(2-acrylamido-2-methyl-1-propane sulfonic acid)[PAMPS=2,000,000]; poly(dimethylamine-co-epichlorohydrin) [PDME];polyaniline (Emarlidine base) [PANI, MW=100,000] and Nafion 117 as a 5%resin solution.

Synthesis of the Carbon-Catalyst Colloidal Suspensions. A carboncolloidal suspension was considered reliable for LBL assembly if thecarbon particulates did not settle to the bottom leaving a gray or clearlayer above them. The settlement test was done after stirring themixture, followed by sonication for one hour, then leaving the carboncolloidal suspension to rest (i.e. no stirring or agitation of any kind)for one day. The carbon colloidal suspension was prepared by dissolving0.004 g of the carbon powder in 1.0 mL of 10.0 mM polymer solution ifthe powder was dry. For the aqueous carbon paste, that was not loadedwith the catalyst, a prior step was taken. The platinum powder (0.02 g)was sonicated in 100.0 mL of 10.0 mM polymer solution for one or twohours to ensure proper dispersal of the catalyst. Afterwards carbonpaste (1.0 g) was added to the 100.0 mL catalyst-polymer solutionfollowed by 1 hr sonication. The following properties indicate the typeof carbon and polymer products that can be used to prepare a suitablecarbon colloidal suspension. The properties of the carbon needed for asuitable collidal suspension include a graphite type that possess lowelectrical resistivity (i.e. <5×10⁻⁴ Ωcm). For example, 10% platinum onactivated carbon was successfully deposited on a PDAC/PAMPS LBL film.

For example, using scanning electron microscopy (SEM), an LPCEsynthesized by mixing carbon paste and Pt powder (size ˜0.15-0.45 μm)showed ˜10-20 μm Pt agglomerates dispersed between carbon particulates.In addition the carbon should be treated or pretreated with a base suchas ammonia solution or its organic derivatives in order to disperse itproperly in aqueous solution. For example, the Hispec3000 powder wasprocessed at pH=10 to 11, the graphite paste is ammonia treatedaccording to manufacturer's specifications, while the untreated drygraphite powder failed to form any colloids. Furthermore, the carbonshould have a small particle size (about 1.0 μm or less) to form goodcolloids as settlement depends on atomic mass according to Stokes law,while sub micron size colloids are needed to fabricate micron size powerdevises. Finally, the carbon should be loaded with nanometer sizecatalyst colloids rather than having the catalyst mixed with it. Forexample, using scanning electron microscopy (SEM), an LPCE synthesizedby mixing carbon paste and Pt powder (size ˜0.15-0.45 μm) showed ˜10-20μm Pt agglomerates dispersed between carbon particulates. Unfortunately,the bigger the platinum colloids the faster they settle down in solutionand their surface area becomes smaller hence less power is generated.

It should be noted that all the carbon colloids used in this study werenot of the submicron size according to the manufacturer's specificationsand SEM. In colloidal suspensions that were prepared but not used acolloidal layer formed on top of the agglomerates indicating asegregation of the small particulates that remained in solution whilethe larger ones settled down. Many commercial electrodes use carboncolloids that are 100 nm or less in size and therefore are expected togive positive tests with most polymer solutions.

Properties of the catalyst. The catalyst could be inert type (e.g.,noble metals Pt, Au, Ru) or active type (e.g., any metal Zn, Cu, Ag).For example, Pt and Al powders were mixed with carbon paste to preparecolloidal suspensions for LPCEs according to the procedure explainedabove. In addition, the catalyst should be properly dispersed insolution using stabilizers such as PVP or suitable polyelectrolytes inorder to prevent agglomeration. For example, Pt colloids weresuccessfully prepared and dispersed in PAA, PSS and PANI solutions.Finally, since Stokes law states the size of the colloidal metalliccatalyst used depends on its atomic mass and since platinum colloidsshould have a maximum size of 60 nm to remain suspended for few days(vida supra), Pt-carbon-polymer colloidal suspension are robust enoughto deposit platinum particulates of only a few microns in size.

Properties of the polymer solution. In general, all strongpolyelectrolytes such as poly(−) PSS, PAMPS, Nafion and poly(+) PDAC,PAH successfully formed carbon colloidal suspensions. Best results wereobtained when Hispec3000 or 10% Pt on activated carbon were treated withPDAC as poly(+) solution and PAMPS as a poly(−) solution at pH=10 to 11.In general, weak polyelectrolytes, such as poly(−) PAA, PAA-co-PAAM andpoly(+) LPEI, yielded best results with the aqueous carbon paste. TheLPEI/carbon paste and PAA/carbon paste colloidal suspensions wereprepared and used in LBL deposition at pH=4 without having to increasethe pH to pH=11. In fact, a PAA/carbon paste suspension lasted formonths. The same polyelectrolytes tended to produce agglomerates withthe Hispec3000 or 10% Pt on activated carbon. At high pH PAA andPAA-co-PAAm yield quality Hispec3000 or 10% Pt on activated carbonsuspensions that can be used in LBL deposition. Using a low pH<2 torender weak polyelectrolytes, such as LPEI, strongly positive was noteffective because at low pH the carbon colloids tend to agglomerate.

LBL deposition to assemble the LPCE. To insure acceptable electricalconductivity and good catalyst loading of both polyelectolytes thepoly(−) and the poly(+) solutions were loaded with carbon and platinum(or catalyzed carbon). This method of colloidal LBL deposition is unlikeany other LBL method in the literature as the colloids are placed inboth the poly(−) and the poly(+) solutions. All previous LBL methodshave the colloid in one beaker and the polyelectrolyte in another.Whether using (+)LPEI(10.0 mM, 100 mL)/C(0.1 g)/Pt(0.02 g) with(−)PAA(10 mM, 100 mL)/C(0.1 g)/Pt(0.02 g) or (+)PDAC(10.0 mM, 100mL)/Hispec(0.1 g) with (−)PAMPS(5 mM, 100 mL)/Hispec(0.1 g) the C/Ptcolloids were deposited on the substrate in every dipping. Thus thegraphite particulates are held in intimate contact and an electronicconductivity up to 2.0 Scm⁻¹ was recorded. Unlike LBL conducting filmsthat use conducting polymers such as poly(aniline) or poly(pyrrol), theLPCE electrical conductivity does not degrade with time. Moreover, X-raySEM analysis showed similar levels of catalyst loading when a Hispec3000powder sample was compared to an LBL deposited sample. However, a 10bilayer LPCE (Hispec3000 type) showed nearly 2.5 times less in platinumloading compared to a commercial E-TEK® electrode. This should not be aproblem because the catalyst layer in the E-TEK® is around 30 μm thickwhile the 10 bilayer LPCE was measured by profilometry to be ˜6 μmthick. The dipping time in the carbon colloidal suspension was 20minutes followed by 2.0 minutes drying and three rinses with pure waterwhere each rinse lasted for 2.0 minutes without any agitation.

From an industrial and economic point of view the cost of preparationand handling the carbon colloidal suspensions should also be considered.We have tested the number of LBL depositions and the total surface areaof the assembled electrodes; a 100 mL solution used over two weeks wascapable of producing LPCEs of a total surface area ˜50 cm². The carboncolloidal suspensions are also recoverable and recyclable. Recovery ofthe carbon and platinum is achieved by evaportation of the solvent(water). This reisolated material, plus an additional quantity ofmaterial, added to account for the lost mass of solids, can be stirredand sonicated in 100 mL poly(+) or poly(−) solution and the processrepeated.

Depositing and testing LPCEs on solid substrates. In order to check onits effectiveness, the LPCE was first deposited on solid substrates,such as platinum, gold, porous stainless steel, and Silicon-100. Goldwas first dipped in a poly(−) colloidal suspension because it has apositively charged surface. After 10 bilayers, the gold surface wascompletely covered with the LPCE. On a Si-100 substrate that wasoriginally covered with 40 bilayers of PDAC/PAMPS, a 10 bilayerHispec3000 based LPCE showed an average thickness of ˜6 μm measured byprofilometry.

Testing the open circuit potential (OCP) of the LPCE. To check on theelectrochemical activity of an LPCE-coated gold electrode [Pt—C/Au] theOCP of bare gold and bare platinum electrodes were tested against astandard calomel electrode (SCE) in a 20 mM H₂SO₄/H₂ (g) purged solutionwith the following results: Pt/0.02M H₂SO₄/H₂(g)//SCE the OCP=+0.32 to+0.359 V; Au/0.02M H₂SO₄/H₂(g)//SCE the OCP=−0.005 to +0.02 V; and for acarbon coated gold electrode: [C/Au]/0.02M H₂SO₄/H₂(g)//SCE theOCP=+0.005 to +0.03 V; while for a carbon/platinum coated goldelectrode: [Pt—C/Au]/0.02M H₂SO₄/H₂(g)//SCE the OCP=+0.345 to +0.347 V.

It is clear that the [Pt—C/Au] yielded an OCP in the same range as anuncoated pure Pt electrode. Similarly, a porous stainless steel SS316Lfilament was coated with the Hispec3000 catalyst to give an OCP=+0.338V. In fact two LPCE-Hispec3000 coated SS316L filaments each purged withH₂ and air respectively yielded an OCP=+0.65 V. When a 10 bilayerLPCE-Hispec3000 coated SS316L filaments were utilized in a galvanic cell(acting as an aqueous fuel cell) where the cathode compartment containedan acidified dichromate solution (10.0 mM Cr₂O₇ ²⁻ in 1.0 N H₂SO₄) as anoxidizer and the anode compartment contained a basic borohydridesolution (10.0 mM BH⁴⁻ in 0.5M NaOH) as a reducer it generated anOCP=+1.63V, while pure platinum electrodes yielded an OCP=+1.58V, andthe uncoated SS316L filament electrodes yielded an OCP=+0.89V. Thedifference in the power generated is 2.0 mWcm⁻² by the LPCE-Hispec3000coated SS316L, 0.088 mWcm⁻² by the uncoated SS316L filament electrodes,and 0.06 mWcm⁻² by the pure platinum electrodes. Results from the powergenerated signify effective platinum loading in the matrix of the LPCE.Note that the aqueous fuel cell (described below) was used to compareperformance because its salt bridge recorded several hundred ohms ofinternal impedance.

Testing the ionic conductivity of the LPCE. The following electrodes ofthe same area were placed in acidic solution at the same distance from abare platinum electrode and the impedance was measured. For a bare goldelectrode: Au/0.02M H₂SO₄/Pt the Z=70Ω; and for a carbon coated goldelectrode: [Au—C]/0.02M H₂SO₄/Pt the Z=72Ω; and for a carbon/platinumcoated gold electrode: [Au—C/Pt]/0.02M H₂SO₄/Pt the Z=72Ω. The resultsshowed a 2Ω increase in impedance, implying that the LPCE exhibits ahigh ionic conductivity when fully wet and an indication of its highlyporous nature.

Depositing and testing the LPCEs on polymer membranes. The LPCE wassuccessfully deposited on membranes such as Nafion112® and PDAC/PAMPScomposite membrane. LPCEs based on carbon paste were deposited directlyon Nafion112® surface while LPCE based on Hispec3000 failed. In thiscase, the Nafion can be functionalized with a few bilayers of PDAC/PAMPSand the Hispec3000 can be successfully deposited on Nafion112® membrane.LPCEs based on the three carbon brands (that is, aqueous carbon paste,Hispec3000, and 10% activated carbon) were successfully deposited on thePDAC/PAMPS composite membrane to form soft “MEA” or catalytic surfaces.The LPCE was found to adhere intimately to the ion conducting membraneeliminating the need to apply hot pressing (at a temperature of about180-200° C. and a pressure of about 70 to 80 atmospheres for about 2minutes) to fabricate the MEA, as is usually needed with conventionalfabrication methods.

Conventional deposition techniques. In some cases, LBL deposition toassemble an LPCE is not necessary. Analogous to conventional methods offabricating the catalyst layer, a polyelectrolyte complex gel replaces aNafion® ionomer solution instead. The slurry of a polyelectrolytecomplex solution plus catalyst ink/PTFE emulsion/stabilizer can bepoured directly onto the composite membrane to form a uniform film whichis then dried and hot pressed. Uniformity and thickness depend on themethod used, such as spin coating, decaling, or spraying. These methodsare faster than LBL deposition but are incapable of thickness control,uniformity, compactness, and performing microcontact printing.

Properties of a Soft Membrane-Electrode-Assembly (MEA). When properlymounted, a soft MEA, running on H₂ and air, produced a stable OCP up to+0.9V similar to any commercial MEAs. The overall impedance Z_(T) (i.e.contact impedance, electronic impedance, and ionic impedance) hence theconductivity of a PDAC/PAMPS soft MEA was measured and compared to bothan E-TEK®/PDAC/PAMPS MEA and a commercial MEA under similar conditions.A commercial MEA gave a Z_(T)=2Ω at RH˜95%. An E-TEK®/PDAC/PAMPS MEAgave Z_(T)=85Ω at RH˜94% and Z_(T)=70Ω during fuel cell operation. Infact, around 60Ω were lost to contact resistance in our E-TEK®/LBL MEAs.With a PDAC/PAMPS soft MEA a Z_(T)=10 to 20Ω was achieved at RH˜95%. Thereason for such a high conductivity resides in the adaptability of theLBL technique. When the PDAC/PAMPS soft MEA is installed dry withoutdosing with a plastisizer solution its Z_(T)=5000Ω at RH˜55%. When thesame soft MEA is dosed with 0.02M H₂SO₄ solution its Z_(T)=250Ω atRH˜60% and Z_(T)=100Ω at RH˜95%. Dosing with a mixture of 0.02MH₂SO₄/2.5% Nafion117® solution yielded a Z_(T)=30Ω at RH˜95% andZ_(T)=10Ω at RH˜99% which is another indication that a soft MEA iscapable of high ionic conductivity.

IIb. Membrane-Electrode Assemblies (MEAs) to Gas Diffusion Layers (GDLs)

The synthesis mechanism is exactly the same as the LPCE assemblydescribed above with the omission of the catalyst in all its forms. Thesize of the particle of carbon or any electronically conducting colloidshould be micron size to ensure good diffusion of gases. GDLs made fromaqueous carbon paste were found to adhere intimately to the precedingcatalyst (i.e. LPCE) layer and provided high electrical conductivity. Aswith the LPCE, the GDL should also be hydrophobic to expel water andallow the diffusion of gases.

IIIa. Devices—Fuel Cells: Polyelectrolyte Multilayer Thin Films as PEMs

We have developed polyelectrolyte multilayer thin films with relativelyhigh ionic conductivity constructed with hydrophilic weakpolyelectrolytes, using either electrostatics or hydrogen bonding as ameans of assembly LPEI/PAA and PEO/PAA. In particular, films constructedfrom linear poly-ethyleneimine (LPEI) and poly(acrylic acid) (PAA),exhibited ionic conductivity as high as 10⁻⁵ Scm⁻¹ at 100% relativehumidity and room temperature; these films also exhibited high ionicconductivity at ambient conditions (50% RH, 25° C.) when prepared andpost-treated with an oligoethyleneoxide/aqueous solution at low pH.Polyethylene oxide (PEO) and PAA alternating thin films were constructedthrough hydrogen bonding interactions, and stabilized with lightcrosslinking, to obtain films with conductivities of 10⁻⁵ to as high as10⁻⁴ Scm⁻¹ at ambient conditions when films were constructed at highsalt concentrations. In both cases, the ionic conductivity was optimizedat film assembly conditions which led to thick polyion bilayers, andloopy, coiled polymer surface conformations. The use of LPEI, PEO andother polymers with a relatively hydrophilic nature greatly enhanced ionconduction in these films.

Herein, ultrathin LBL polymer films were utilized as the top, continuousthin film layer of a composite polymer membrane. A nonconducting poroussubstrate was chosen as the intermediate, or core layer, in thisconstruction. Here we use a commercially available polycarbonateNucleopore® membrane, which has a total thickness of 6.0 μm, and regularpores of 100 nm diameter, as the base membrane. The pores in theNucleopore membranes are created via gamma irradiation, and aretherefore cylindrical and regular in geometry, with single porestraversing the entire thickness of the film. This simple geometryprovides a clear path of ion transport, and thus the pores act as theconducting channels between the two LBL film/electrode assemblies.

The Nucleopore base was then plasma treated and, in all cases, a 40bilayer thin film of the LBL polymer pair of interest was constructedonto the surface of the membranes. By selecting the molecular weight andionic strength or solution pH, it was possible to tune the hydrodynamicradius of the polyion of interest in solution. If the hydrodynamicradius is relatively small compared to the pore diameter, a multilayerthin film will form within the pores; however, if the molecular weightand solution conditions are such that the polymer chains are larger thanor close to the pore size of interest, assembly results in bridging ofthe pore diameters, and the film builds up only on the outer surface ofthe membrane. The molecular weight of the polymers used was such thatthe pore diameters were bridged, and the multilayer film existed only onthe base membrane surface. This fact was confirmed by scanning electronmicroscopy (SEM), as shown in FIG. 1(a-c). A top view of the compositemembrane, for which a part of the film was removed with a stylus,indicates clearly the presence of open pores underneath the top,uniform, pore free LBL film layer. Cross-sectional images in FIG. 1(b-c)indicate that the LBL film is present only on the top surface, and thata symmetric film forms on both sides of the membrane.

Peak areas from transmission FTIR that were measured by referring to the—COOH stretch in PEO/PAA, PEO/PMAA both at (1878-1577 cm⁻¹), andLPEI/PAA at (1762-1660 cm⁻¹); or the SO₃H stretch in LPEI/PSS at(1058-1018 cm⁻¹), and finally the —CONH₂ stretch in PAN/PAMPS at(1637-1536 cm⁻¹) or PAN/PAAcoPAAm at (1632-1535 cm⁻¹) were used todetermine the relative amount of the PE material deposited on bothsubstrates. Both the Si-100 and the IR transparent Nucleopore substratesshowed an average difference of only ±5% an indication that the amountof PE material is similar on both surfaces. Moreover, AC impedance andgalvanostat measurements gave much lower resistances than expected ifthe PE material were to fill the pores. For example, the conductivity ofthe PEO/PAA LBL film can reach a maximum of 10⁻⁴ Scm⁻¹ at RH=100%.Calculations show that a 20 μm thick composite membrane (Table 1) isexpected to have a resistance of about 40Ω if the pores are filledcompletely with PEO/PAA film. Unexpectedly, the result obtainedexperimentally was 5 to 13Ω, indicating that the pores and the film werefully soaked. The result of this composite film geometry is that duringfuel cell operation at high humidity, the membrane allows the passage ofwater and ions, ultimately resulting in water-filled pores between theLBL films; the electrodes contacting the LBL films, which serve as abarrier to gases across the membrane, and a gateway for ions generatedat the electrodes.

Because of differences in the nature of the multilayers achieved withdifferent polyion pairs, the thickness of the LBL film covering thepores ranged between about 1 μm to about 5 μm on one side of themembrane. The thicknesses as determined by profilometry, and thecorresponding pH of assembly for all of the polyion pairs studied hereare shown in Table 1; in all cases, the same adsorption conditions wereused for both polyions in a given polyion pair. Table 1 also containsthe FC open circuit voltages and a summary of the resistance across thecomposite membrane as measured at 90 to 95% relative humidity and atemperature of 23±1° C. using AC impedance spectroscopy. The resistanceas determined from the galvanostat was assumed to be that at 100%relative humidity and not at the humidity of the chamber (50 to 60%)because the fuel cell generates water that is soaked up by the compositemembrane. The corresponding conductivities were determined using theequation: $\begin{matrix}{\sigma = \frac{t}{R \cdot A}} & (1)\end{matrix}$where σ is the conductivity (Scm⁻¹), t is the thickness (cm), R is theresistance (Ω), and A is the area of the electrode (cm⁻²). Each of thesystems shown below were optimized to achieve optimal ion conductivityvia variations in the ionic strength and pH of assembly. It is clearthat these systems produce unusually large bilayers, ranging inthickness from 26 to 120 nm/bilayer.

Fuel Cell Measurements and Ionic Conductivity. Beyond providing amechanical support and added stability to the LBL film, the porousframework of the Nucleopore membrane provides unhindered protonconductivity. One must consider the role of the multilayer thin film andits ionic resistance with respect to the electrolyte filled pores of theporous polycarbonate base. TABLE 1 Total thickness Resistance ResistanceFuel cell Open circuit Membrane (μm)/pH AC-Impd. Galvanostatconductivity potential System assembly (Ω) (Ω) (S cm⁻¹)^(d) OCP (V)LPEI/PSSM  6.0/4.0 1400 2500  9.6E−07 0.92 LPEI/PSS 2.4^(a)/2.7 114  1361.23E−05 0.87 LPEI/PAMPS 2.1^(a)/2.5 c  104 1.56E−05 0.82 PEO/PMAA8.5^(a)/2.0 91  145  2.0E−05 0.87 LPEI/PAA 6.4^(a)/4.0 65  76^(e)3.26E−05 0.85 PDAC/PAMPS^(b) 2.2^(a)/6.0 28  45^(e) 3.64E−05 0.89PDME/PAMPS^(b) 2.2^(a)/6.0 24  30 5.46E−05 0.64 PEO/PAA 97^(a)/2.0˜1.5-5  13^(e) 0.00024 0.88 Nafion112 ® 50 ˜0.5-1   6.7 0.0015 0.95^(a)Only includes the thickness of the LBL film material on both sidesof the pore.^(b)Polyelectrolyte solutions contained 0.5M NaCl.c delaminated.^(d)Area = 0.5 cm⁻²; Thickness = Nucleopore + LBL films;Resistance→Galvanostat.^(e)determined using the fuel cell equation 3.11 in Larminie, J.; Dicks,A. Fuel Cell Systems 2000, Wiley, New York.

Ionic Conductivity Analysis. The advantage of using a track-etchedpolycarbonate membrane is the straight pores that traverse the membranefrom one side to the other, thus making theoretical manipulation easier.What starts the conduction process is a short burst of ions at thesurface of the electrode that disturbs the overall charge neutrality ofthe system. The protons that propagate across the bulk of the solutionby the Grotthuss mechanism would do the same across the hydrophilicexchanger sites of the LBL films. An exchanger site is an ion-pair sitethat is surrounded by a cluster of water molecules that are in a dynamicequilibrium with the surrounding water pools. Farhat, T.; Yassin, G.;Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623; Losche, M.;Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998,31, 8893. The plane of scatter of ions drift across these sites with acurrent density (A cm⁻²) such that:j=v·c·F  (2)where v (cms⁻¹) is the drift velocity of the scattering plane ofprotons, c (molcm⁻³) is the concentration of ions, and F is the Faradayconstant (about 96,500 Cmol⁻¹). In the presence of an electric field E(Vcm⁻¹) the current density is also defined as:j=σ·E  (3)Combining equations (2) & (3), the conductivity can be defined in termsof the drift velocity of the plane of scatter of ions. $\begin{matrix}{\sigma = {v \cdot \frac{c \cdot F}{E}}} & (4)\end{matrix}$Using Monte Carlo algorithm the drift of the plane of scatter of ionscan be monitored across a fixed distance by counting the number ofcycles required for the plane to drift from one electrode to the other.Lui, J. S. Monte Carlo Strategies in Scientific computing, NY, Springer© 2001; Madras, N. Monte Carlo Methods, Providence R.I., AmericanMathematical Society © 2000. In a 2D coordinate space of 400×200 pointsa random distribution of a population of hopping sites [5000,representing either water molecules or “hydrated ion pair” exchangersites] would be used as a ground for a more dilute set of conductingpoints [50 or a 100, representing the protons] to propagate across the2D system. A hopping point can hop in all directions in an asymmetricway (FIG. 2 b) with a tendency to move in the direction dictated by theelectric field. It should be noted that interactions based on quantummechanical treatments, which affect the hopping time, were neglected andif a conducting point strikes a site the hopping attempt is consideredsuccessful. Normalized values of the drift velocity plotted against thefraction of exchanger sites whether inside the bulk of the electrolyteor within a pore surrounded by a hydrophobic matrix is shown in FIG. 2a. In both cases the values are overlapping which indicates that theconductivity won't be affected by porosity.

Since proton conduction can largely be related to the concentration ofthe hopping sites (i.e. the degree of hydrophobicity) then MCA showsthat the drift velocity would decrease linearly (plot not shown) as theconcentration, expressed here as the % population of sites, wasdecreased FIG. 2 c. Fortunately, MCA proves that by having aqueous porestraversing the LBL composite membrane a maximum conductivity can beachieved (i.e. σ˜0.015 Scm⁻¹ corresponding to a 20 mM H₂SO₄ solutioncompared to a σ˜10⁻⁴ Scm⁻¹ of a PEO/PAA matrix).

AC Impedance Measurements of Polyion Pair Composite Membranes. Resultsobtained by AC impedance on fuel cells running under dry conditions(RH<40%) while the humidity was ramped up to operating conditions areshown in FIGS. 3 a, 3 b. The complexity of the Nyquist plots reflectsthe structure of the composite membrane. Multiple semi-circles at lowhumidity indicated a system that is composed of Randle's cells connectedin series. As the humidity was ramped up the impedance decreased becausethe pores of the polycarbonate membrane filled with water. As the fuelcell was further purged to operating conditions the high frequencysemi-circle referring to the polarization resistance of the porouselectrodes interface or pore's impedance, Z_(p), disappeared and thepolarization resistance of the LBL membrane-electrode interface Z_(m)impedance was the only feature left (FIG. 3 b). The tail of the Nyquistplot, which was assigned as a Warberg Constant-Phase-Element (CPE),indicates that ion diffusion through a microporous structure is indeedoccurring in all measurements. The circuit shown in FIG. 4 a was used tosimulate the behavior of the fuel cell under different humidityconditions using the following equation: $\begin{matrix}{Z_{T} = {Z_{p} + Z_{m}}} & (5) \\{Z_{p} = \frac{\frac{1}{R_{p}} - {{j\omega}\quad C_{p}}}{\left( \frac{1}{Rp} \right)^{2} + \left( {\omega\quad C_{p}} \right)^{2}}} & (6) \\{{Z_{m} = \frac{R_{m} - {X^{\varphi}\cos\quad\beta} - {j\left( {{\omega\quad C_{m}R} - {2\quad\omega\quad C_{m}R_{m}X^{\varphi}\cos\quad\beta} + {\omega\quad C_{m}X^{2\varphi}} + {X^{\varphi}\sin\quad\beta}} \right)}}{\left( {{\omega\quad C_{m}X^{\varphi}\sin\quad\beta} + 1} \right)^{2} + \left( {{\omega\quad C_{m}R_{m}} - {\omega\quad C_{m}X^{\varphi}\cos\quad\beta}} \right)^{2}}}{{\beta = {\frac{\pi}{2}\varphi}},{X = \frac{1}{\omega\quad C_{W}}}}} & (7)\end{matrix}$where R_(p) is the pore resistance, C_(p) is the pore capacitance, R_(m)is the membrane resistance, C_(m) is the membrane capacitance, φ is thefractional exponent, C_(W) the CPE capacitance, and X^(φ) is the WarbergCPE reactance.

We have considered a contribution from two Randle's cells connected inseries. Both the Z_(m) and Z_(p) depend on the concentration of H⁺ ionsand on the water uptake, i.e. the degree to which the channels and poolsof water interconnect across the composite membrane. The simulatedNyquist plots that utilized the circuit in FIG. 4 a showed two effects:(1) A drop in R_(p) and R_(m) as humidity is increased leads to anoverall drop in the total impedance reflected by a shrinkage in the sizeof the Nyquist plot; and (2) A sharp drop in R_(p) (pores fully soaked)diminished one semi-circle (i.e. the pores), leaving one Randle cellthat corresponds to Z_(m) at maximum humidity.

Galvanostat Measurements of Polyion Pair Composite Membranes. After eachfuel cell operation the resistance of the MEA was measured using ACimpedance and compared to values of resistance calculated from the opencircuit potential [OCP] of the fuel cell, and the junction potentialsrecorded for specified applied currents, where: $\begin{matrix}{{R_{j} = \frac{{OCP} - V_{j}}{I_{j}}}{{j = 1},2,{3\quad\ldots}}} & (8)\end{matrix}$Equation (8) is a simplified form of the fuel cell equation. Larminie,J.; Dicks, A. Wiley, New York 2000. Simulations using the power densityplots of LPEI/PAA showed that results obtained by eq. (8) and the fuelcell equation for R≧10Ω are nearly similar. For all composite membranes,we conclude that the system under study is a combination of H⁺ ionspropagating across a nonporous LBL film phase into a porous aqueousphase thus creating a “nano salt bridge”. Values of the resistanceshowed that when the fuel cell is operating even at low RH of ˜50% to60% the composite membrane performs as if it is wet or at a RH of about100%. It is probable that dynamic hydration equilibrium is occurringbetween the water produced at the cathode, the LBL film, and the waterwithin the pores.

Fuel cell performance. In order to compare the performance of ourcomposite membranes to the classical PEM (Nafion112®, both types ofmembrane were moderately pressed between two commercial Pt/C (ELAT®)electrodes to fabricate the MEA. The MEA was then sandwiched between twohomemade bipolar plates that allowed gas diffusion and acted as currentcollectors. The whole assembly was enclosed inside a two-chambermanifold that contains the fuel gases, and through which the pressure,temperature, and humidity can be controlled at ambient conditions. Fromthe brief description of the FC design it should be noted that all thefuel cell systems studied were run under a regime of non-optimizedperformance. This design was chosen to minimize any interferences fromoutside factors such as ionomer wicking inside the composite membraneand thermal or mechanical degradation if press baked or operated at hightemperature (> about 60° C.) and pressure (> about 10 psi).

Two control experiments were made to analyze the performance ofcomposite membrane. First, a Nucleopore® membrane sandwiched between twoELAT electrodes should show the highest conductance, hence the maximumpower delivery, because the conducting protons are not impeded by theLBL film sealing the pores. A fuel cell running on an uncoatedNucleopore® membrane delivered a maximum power density of 9.6 mWcm⁻² andits OCP was 0.8V, which decreased with operation to 0.56V due touncontrolled fuel cross over, thus illustrating the important role ofthe LBL film in regulating fuel cross over, and the need for thisintermediate layer in the MEA. Second, a home-built E-TEK®/Nafion112®fuel cell system was capable of delivering a maximum power of 43 mW cm⁻²at a RH≧80% and a temperature of 25° C. A strong flow of hydrogen wasmaintained to obtain high humidity with P_(air)=4.0 psi, and theoperation time was up to two hours. If the humidity was decreased to arange of 60% to 50%, the power dropped to ˜23 mW cm⁻² while the voltagecontinuously decreased. When the RH was dropped below 50% theE-TEK®/Nafion112® fuel cell system simply collapsed.

We have organized the LBL fuel cell systems into two categories wherethe low-power systems are shown in FIG. 5, and the high-power systems inFIG. 6. At ambient conditions, the low-power couples LPEI/PAMPS,LPEI/PSSM, PEO/PMAA, and LPEI/PSS did not deliver an electrical powerhigher than 4 mWcm⁻², while the high power couples PDME/PAMPS,PDAC/PAMPS, LPEI/PAA, and PEO/PAA delivered greater than 5 mWcm⁻². Bycomparing the systems in FIG. 5 & FIG. 6 the effects of film structureand composition on the performance of the fuel cell can be discussed.Results in Table 1 show that MEAs containing different PE couples showdifferent internal resistances, and in general, those that are lesshydrophobic are more conductive to H⁺. However, thehydrophilic-hydrophobic factor is not the only important factor, but thestability of the LBL film and the size of the polymer molecule may alsoplay a role in power delivery. For example, wet LBL films of LPEI/PAAdeposited on gold-coated substrates have a much higher conductivity thanLPEI/PSS films (5×10⁻⁶ Scm⁻¹>>1.2×10⁻⁸ Scm⁻¹). The difference isattributed to the higher hydrophilic character of PAA compared to PSS.One might assume that the conductivity of PSSM would therefore have anintermediate value of 0.9×10⁻⁷ Scm⁻¹) as PSSM contains a mix of a 3:1ratio of sulfonate to carboxylate groups. By referring to the powerplots in FIG. 5 and FIG. 6, it is clear that the power delivered by theLPEI/PAA fuel cell (˜5.5 mWcm⁻²) is higher than that of LPEI/PSS (˜3.0mWcm⁻²) for the reasons stated. Unexpectedly, the power delivered by theLPEI/PSSM system (0.16 mWcm⁻² at I=0.4 mA cm⁻²) is not intermediate, butlower than that of the more hydrophobic LPEI/PSS. Although the LPEI/PSSMfilm is 2.5 times thicker than the LPEI/PSS film, this difference doesnot explain the 18.8 times difference in power density (i.e. 3.0:0.16mWcm⁻²). Both the LPEI and the PSSM polymer molecules possess a lowaverage molecular weight that allows the PE couple to penetrate throughthe pores and form a few bilayers on the inner walls of the pore, andthus leading to greater resistance to proton mobility, and hence muchlower power density.

Comparing LPEI/PAMPS (3.6 mWcm⁻²), FIG. 5, and PDAC/PAMPS (˜11.5mWcm⁻²), FIG. 6, the former couple might have been expected to producemore power because LPEI is more hydrophilic than PDAC. Under operatingconditions PDAC/PAMPS showed a better performance probably because theLPEI/PAMPS film was viscous and unstable, allowing fuel cross over, andunder the severe environment of the fuel cell, the film delaminated andleaked gases. The PDME/PAMPS (6.0 mWcm⁻², plot not shown) system wasexpected to perform better than PDAC/PAMPS because PDME is lesshydrophobic than PDAC due to the presence of an OH group on the polymerbackbone. However, it yielded a lower OCP (0.81 degrading to 0.64V) anda maximum power density of 6.0 mWcm⁻², similar to the LPEI/PAA system.In general, films that were suspected of fuel crossover yielded a lowOCP. A PDME/PAMPS fuel cell operating at 0.46V and a current of 12.0mAcm⁻² did not show any degradation in its voltage even after 30minutes. The most stable couple among the low power LBL fuel cells wasthe LPEI/PSS system which after a few hours of operation kept deliveringthe initial power or even higher, up to 3.7 mWcm⁻².

Among the PE couples studied, the PEO/PAA system delivered the highestpower (16.6 mWcm⁻²), which was nearly 50% the performance of theE-TEK/Nafion112 fuel cell operated at RH>80% in our laboratory, andclose to many commercial monocells at 20 mWcm⁻² operated under the sameconditions. The PEO/PAA system is followed by the PDAC/PAMPS system at11.6 mWcm⁻² and the LPEI/PAA system at ˜5.5 mWcm⁻². The PDAC/PAMPS filmhad higher conductance than LPEI/PAA film because its thickness isnearly 2.5 times lower at an equivalent number of layers. All threesystems were stable especially PEO/PAA that was tested, after a two hourexperiment, by operating at half power for 10 minutes with hardly anychange in potential. Additionally, the capability of the PEO/PAA system,as well as the other LBL fuel cell systems, to operate normally atRH=50% is a major advantage over the Nafion112® membrane used. Forexample, separate tests on the LPEI/PAA (5.5 mWcm⁻²) and LPEI/PSS (3.7mWcm⁻²) fuel cell systems operated using a dry flow of H₂ and air atRH˜5% were successfully performed. The significance of having the LBLfilm deposited on the Nucleopore platform and not directly on the ELATelectrodes was demonstrated when we tested an LBL PEO/PAA film directlydeposited onto the ELAT electrodes, where the latter were separated by acomposite membrane made up of only 10 bilayers of PEO/PAA. The powerdelivered by this system (1.84 mWcm⁻²) was extremely low, and wasattributed to the permeation and probably precipitation of thepolyelectrolytes within the pores of the electrode during assembly, thuscreating resistances within the electrodes to proton conductivity aswell as a hydrophilic medium that absorbs water into the electrodes,blocking the passage of fuel. These early results indicate that thePEO/PAA system is a promising one for actual fuel cell applications.

Therefore, there are three important factors that determine the powerperformance of a PE-LBL fuel cell. To simplify, consider the three PEcouples PDAC/PAMPS, PDAC/PSS, and LPEI/PSS. The PDAC/PSS couple (σ<10⁻⁹S cm⁻¹) is characterized by a strong hydrophobic electrostaticinteraction between the tertiary ammonium group [—N—⁺] of PDAC and thesulfonate group [—SO³⁻] of PSS leaving no stations for proton conductionacross the PE segments. Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff,J. B. Langmuir 1999, 15, 6621-6623; Dubas, S. T.; Schlenoff, J. B.Macromolecules 1999, 32, 8153-8160. As a result, no power was deliveredby this system. Replacing the PDAC by LPEI with a polycation backbonethat constitute secondary ammonium groups [—NH—⁺] that are pH dependentand hydrophilic resulting in weaker interactions with the [—SO³⁻] ofPSS. Yoo, D.; Shiratori, S. S.; Rubner, M. F. 1998, 31, 4309-4318;Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206-10214.Consequently, proton exchange can progress moderately across thefunctional groups. When PDAC/PAMPS is compared to LPEI/PSS, the formerdelivers about 4 times the power even though both LBL films have thesame thickness. Although, in PDAC/PAMPS there is a strong hydrophobicelectrostatic interaction between the tertiary ammonium group [—N—⁺] ofPDAC and the sulfonate group [—SO³⁻] of PAMPS; however, PAMPS differsfrom PSS in possessing the amide group [—CO—NH₂—] that is stronglyhydrophilic. DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15,1165-1173. There is a possibility that the amide group acts as a spongefor the water generated at the cathode thus enriching the LBL film withwater pools and channels across which the hydronium ions can propagate.

The impact of hydrophobicity clearly appears when comparing the PEO/PAA(16.6 mWcm⁻²) and PEO/PMAA (2.64 mWcm⁻²) fuel cell systems. The PEO/PAAfilm is nearly the same thickness as the PEO/PMAA film, yet the PEO/PAAsystem was capable of delivering a maximum electrical power around 16.6mWcm⁻², which is 6 times higher than that of PEO/PMAA. The PMAA is morehydrophobic than PAA because it has an additional methyl group in itsrepeat unit. Among the PE couples studied, the PEO/PAA system deliveredthe highest power, which was nearly 50% the performance of theE-Tek/Nafion112 fuel cell operated in our laboratory, and close to manycommercial monocells at 20 mWcm⁻² operated under the same conditions.However, the capability of the PEO/PAA system, as well as the other LBLfuel cell systems, to operate normally at RH=50% is a major advantageover the Nafion112 membrane. For example, separate tests on the LPEI/PAA(5.5 mWcm⁻²) and LPEI/PSS (3.7 mWcm⁻²) fuel cell systems operated usingdry flow of H₂ and air at RH˜5% were successfully performed. Thesignificance of having the LBL film deposited on the Nucleopore and notdirectly on the ELAT electrodes was demonstrated when we tested an LBLPEO/PAA film directly deposited onto the ELAT electrodes, where thelatter were separated by a composite membrane made up of only 10bilayers of PEO/PAA in order to maintain continuity. The power deliveredby this system (1.84 mWcm⁻²) was low, and was attributed to thepermeation of the polyelectrolytes within the pores of the electrodeduring assembly, thus creating large resistances within the electrodesto proton conductivity.

In terms of power delivery, the PEO/PAA system is followed by thePDAC/PAMPS system at 11.6 mWcm⁻² and the LPEI/PAA system at ˜5.5 mWcm⁻².The PDAC/PAMPS film had higher conductance than LPEI/PAA film becauseits thickness is nearly 2.5 times less, although FTIR showed that bothfilms have similar water content by referring to the OH stretch at 3500cm⁻¹. Most of the fuel cell systems were operated for at least two hoursbefore a complete shut down. Repetitive isolation of the FC from itsload did not affect its performance. The average time taken for the FCto regain its OCP or an operating voltage when switched on was oneminute. Our simple design did not use serpentine bipolar plates thatallow intimate flow of gases close to the electrodes to increaseefficiency and remove water; therefore, after prolonged operation, adisassembled MEA was found flooded with water between the compositemembrane and the cathode. Moreover, the Nucleopore membrane hadperformed its role in providing support to the LBL film, but the thermaland mechanical stability of these polycarbonate membranes are less thanoptimal. In the fuel cell module, the Nucleopore membrane is subjectedto some compression and excessive heat that in a number of fuel cellsamples caused cracks in the “composite membrane.” Different supportmembranes with more optimal mechanical and thermal properties anddifferent pore structures are currently being investigated in this work.

IIIb. Devices—Soft Polymer Electrolyte Fuel Cells (SPEFC)

Unlike conventional PEFCs, where the MEA is fabricated by hot pressing,the MEA of a “soft-PEFC” is synthesized in one process that does notrequire hot pressing and gasketing (if the membrane and the electrodesare separate). Using an automated dipper, the process starts byalternately dipping a customized porous-insulating support, FIG. 7, inpoly(+)/poly(−) solutions until the required IEM thickness is assembled.The solutions that make up the IEM can then be replaced by the electrodecolloidal suspensions to deposit the cathode and the anode. Theelectrode colloidal suspensions are in turn replaced by the “gasdiffusion layer” colloidal suspensions to deposit the GDL. If a metallicfilm, acting as a current collector, can now be applied on both sides bysputtering or spraying then a complete soft fuel cell is assembled.Apart from the metallic contacts, we have discovered the aboveexperimental procedure for soft fuel cell assembly.

There were no limitations on the soft fuel cell assembly when strongpolyelectrolytes (i.e. pH independent such as PDAC, PAMPS) were usedthrough out the process. However, if weak polyelectrolytes (i.e. pHdependent such as LPEI, PAA, PAA-co-PAAm) were used to assemble the IEM,then the pH of the carbon colloidal suspension should not exceed 4. Forexample, carbon colloidal suspensions of PDAC/PAMPS maintained at pH=11cannot be used to deposit LPCEs on an LPEI/PAA or PEO/PAA membranes thatbreak down at pH>4. In this case, carbon colloidal suspensions of eitherPDAC/PAA-co-PAAm or LPEI/PAA maintained at pH=4 can be successfullyused.

Soft fuel-cells can be stacked according to conventional gasketingmethods when the soft fuel cells are assembled on insulating-poroussupports. Alternatively, soft MEAs can be stacked on multipleinsulated-porous-metallic supports that act as GDLs and currentcollectors. In this case, stacking does not involve any gasketing ormechanical parts and the whole stack of MEAs is assembled in oneprocess. The LBL process starts with the support executing multipledipping in the poly(−)/poly(+) slurry of the electrodes until thedesired thickness is achieved. The thickness of the electrodes isdetermined by the power delivered, the size of the colloids, and theseparation between the supports. After building the electrodes, the gapbetween the electrodes can be filled by an LBL film (with or without acolloidal layer that mimic the porous membrane). The whole stack canthen be immersed in an ionomer and dried (e.g., Nafion117®) to fill anylose gaps for proton conduction. Finally, to insure tight seal againstgas leakages the whole stack is dipped in a resin solution and dried.FIGS. 8 & 9 show a schematic of the soft stack design. In a thirdembodiment, soft MEAs can be stacked on top of each other by followingthe same procedure described above, but starting with a metallic supportrather than an insulating porous support to form one complete fuel cell.Keeping the whole setup in position but reversing the dipping procedure(i.e. GDL, Electrodes, Membrane) two new soft fuel cells self assembleon the original. The process can be repeated to form a “soft fuel-cellstack”. The whole stack can then be immersed in an ionomer and dried(e.g., Nafion117®) to fill any lose gaps for proton conduction. Finally,to insure tight seal against gas leakages the whole stack is dipped in aresin solution and dried.

IIIc. Devices—Soft Carbon-Polymer Galvanic Cells and Batteries (SCPGC)

The design of a soft fuel-cell can be adapted to design carbon-polymerbased solid-state galvanic cells and batteries, FIG. 10. Unlikeconventional types the SCPGC can be tailored to be ultra thin andextremely small (i.e. be applicable to micro-power systems). Also unlikeconventional solid-electrolytes, in which only one type of ion (eithercation or anion) is predominantly modile and conducts electivity, themembranes of the instant invention can be fabricated to conduct protons,cations and anions. In addition, since the LPCE possesses a largesurface area, the MEA can be a potential candidate for state-of-artsuper-capacitors.

As with soft fuel-cells, the same type of composite membrane, part ofthe electrolyte phase, can be used as a separator or a salt bridge. Thecomposite membrane is best assembled from water-soluble macromolecules(i.e. all types of polyelectrolytes e.g., LPEI/PAA, PEO/PAA, PAAm/PAA,PAAm/PAA-co-PAAm, PDAC/PAMPS, PDAC/PAA-co-PAAm) that possess high ionicconductivity. Since LBL films can be capped and stratified, a half-cellcan be assembled by depositing a suitable polyelectrolyte couple (e.g.,LPEI/PSS) directly onto the electrode then dosing the film with therequired salt ion such as CuSO₄, NH₄Cl, and ZnCl₂.

LPCE electrodes can be of the inert (insoluble) type where the carbonelectrodes are pure or embedded with noble metals, such as platinum,palladium, and gold. Both types of electrodes were assembled. The LPCEelectrodes can also be of the active (soluble) type embedded with metalcolloids such as copper, zinc, and silver or salt colloids such as PbO₂,and MnO₂ which allows it to generate electricity.

IIId. Devices—Soft Carbon-Polymer Electrochemical Cell (SCPEC)

The design of a soft fuel-cell can be adapted to design carbon-polymerbased solid-state electrochemical cells. An electrochemical cell is usedto monitor electrochemical processes by allowing currents to passthrough rather than generating them. The major difference from the SCPFCis to have a passage for electrolytic solutions through the poroussupport in order to dose the analytical cell. Unlike conventional types,the SCPEC can be tailored to be ultra thin and extremely small (i.e.application to micro-electro-analytical systems) thus a potentialcandidate for state-of-art detectors. As with the fuel cell, thecomposite membrane is best assembled from water-soluble macromolecules(i.e. all types of polyelectrolytes e.g., LPEI/PAA, PEO/PAA, PAAm/PAA,PAAm/PAA-co-PAAm, PDAC/PAMPS, PDAC/PAA-co-PAAm) that possess high ionicconductivity. One of the LPCE electrodes must be of the inert(insoluble) type to represent the counter electrode. The LPCE counterelectrode can be pure graphite colloid or graphite colloid embedded withnoble metals such as platinum, palladium, and gold. For the workingelectrode the LPCE electrodes can be either inert or active (soluble)type where the graphite colloid is embedded with metal colloids, such ascopper, zinc, or silver.

IIIe. Devices—Micro-Electrochemical Systems

Unlike conventional electrochemical systems, such as electrochemicalcells, fuel cells, and galvanic cells the LBL electrochemical systemscan be miniaturized using methods such as micro-contact-printing.Conventional microfuel cells made using expensive lithographictechniques still need a relatively large PEM to separate themicroelectrodes. Holladay, J. D.; Jones, E. O.; Phelps, M.; Hu, J. J.Power Sources 2002, 108, 21-27.; Lee, S. J.; Chang-Chien, A.; Cha, S.W.; O'Hayre, R.; Park, Y. I.; Saito, Y.; Prinz, F. B. J. Power Sources2002, 112, 410-418.; Maynard, H. L.; Meyers, J. P. J. Power Sources2002, 109, 76-88. Therefore, it would be difficult to assemble a singlemicrofuel cell using lithography, a job that is made easy using the LBLtechnology.

Polyelectrolyte carbon colloidal suspensions that make up the LPCE donot assemble well on hydrophobic or weakly bonding surfaces that preventelectrostatic interactions or hydrogen bonding. For example, a surfacefunctionalized with long-chain alkyl thiols, weakly bondingpolyelectrolyte, or hydrophobic polyelectrolytes prevents the depositionof carbon colloids at these sites. If part of the surface were treatedwith a suitable LBL film the LPCE would assemble over the LBL film only.On a micro-fluidic substrate a micron size electrochemical system can beassembled with its XY coordinates specified at a particular point on thesurface provided that a channel allows the delivery of the electrolyteor the fuel, FIG. 11. For example, in the aqueous fuel-cell systemdescribed above, the liquid fuel can be easily delivered by capillaryaction through microfluidic channels.

IIIf. Devices—Catalytic Converters

The ability to choose the required particle size of the catalyst (i.e.platinum or any suitable metal plus its carbon support) as well as thesize of the polymer (i.e. the appropriate molecular weight) can beutilized to fabricate catalytic converters. The polymer/carbon LBL filmscan either be deposited on a porous framework with longitudinal tubes,FIG. 12. Or the LPCE can be built thick and porous enough in order toallow the porosity of the particles to mimic the porous support usuallyused in catalytic converters, FIG. 13. For example, a 40 bilayer LPCE ofPDAC/PAMPS 10% platinum on activated carbon was successfully depositedon a porous Nucleopore® support with a thickness up to 30 μm.

IIIg. Devices—Other Applications

A solid state hydrogen pump has all of the same problems as a fuel cell,without the presence of a water forming reaction at the cathode. As witha fuel cells, eliminating the need for a humidifier will lead to asmaller, simpler, and lighter system.

An electrolyzer, especially one designed to produce hydrogen and oxygenfrom water offers a different set of problems. A water electrolyzercontains essentially the same parts as a fuel cells but the polarity isreversed, as are all of the electrochemical reactions. Instead ofgenerating electricity and water from hydrogen and oxygen, it produceshydrogen and oxygen from water and electricity.

In an electrolyzer there is always water present to keep the membranehydrated. The problems arise in the electrodes and on the gas outletside. Because liquid water is present in the same compartment that gasis being generated in, the gas outlet flow will nearly always be twophase with a large quantity of water being carried out with the gas.

A more fundamental problem arises in the electrodes. Since maximumcurrent efficiency requires that liquid water be in contact with themembrane, at least one of the electrodes must be hydrophilic. While ahydrophilic electrode is best for the membrane, it tends to impede gasbubble formation and gas removal. If the water is supplied directly tothe membranes fully hydrophobic electrodes could be used, to maintainefficient gas evolution.

These problems are further exacerbated in a regenerative fuel cell.Since a regenerative fuel cell by definition must operate in turn asboth an electrolyzer and a fuel cell, using hydrophilic electrodes thatproduce effective operation in a liquid water environment forelectrolyzer operation virtually guarantees electrode flooding duringfuel cell operation. If operation with liquid water present in theelectrode compartment can be avoided, then hydrophobic electrodes canfunction well in both modes.

One method that has previously been proposed for directly humidifying aproton exchange membrane is the inclusion of water conducting wicks aspart of the membrane structure. While this method has someeffectiveness, the amount of flow that can be achieved through themembrane is limited. A further drawback to the wicks is that they relyon wetting to promote flow. This precludes their use to introducenon-aqueous streams into the proton exchange membrane. In addition, thewicks act as filtering elements to remove any particles in the stream.This limits their use to systems with pure water, or where care is takento prevent the solution from becoming saturated and beginning toprecipitate.

Electrochemical water desalination or clean-up systems based on theelectroosmosis occurring in a hydrogen pump has some additionaldifficulties other than those noted above for a simple hydrogen pump.This type of system uses the fact that every proton passing through themembrane carries water with it, typically about two water molecules perproton. In devices described previously, the hydrogen and water to bepurified are fed into the cell together as a solution saturated withhydrogen. Since the solubility of hydrogen in water is low, the currentdensity is limited to a relatively low value. A low current densityproduces a low water purification rate.

DEFINITIONS

For convenience, certain terms employed in the specification, examples,and appended claims are collected here.

The term “electrolyte” as used herein means any chemical compound thationizes when dissolved.

The term “polyelectrolyte” as used herein means a polymeric electrolyte,such as polyacrylic acid.

The term “pH” as used herein means a measure of the acidity oralkalinity of a solution, equal to 7, for neutral solutions andincreasing to 14 with increasing alkalinity and decreasing to 0 withincreasing acidity.

The term “pH dependent” as used herein means a weak electrolyte orpolyelectrolyte, such as polyacrylic acid, in which the charge densitycan be adjusted by adjusting the pH.

The term “pH independent” as used herein means a strong electrolyte orpolyelectrolyte, such as polystyrene sulfonate, in which the ionizationis complete or very nearly complete and does not change appreciably withpH.

The term “K_(α)” as used herein means the equilibrium constantdescribing the ionization of a weak acid.

The term “pK_(α)” as used herein means a shorthand designation for anionization constant and is defined as pK_(α)=−log K_(α). pK_(α) valuesare useful when comparing the relative strength of acids.

The term “multilayer” as used herein means a structure comprised of twoor more layers.

For purposes of this invention, the chemical elements are identified inaccordance with the Periodic Table of the Elements, CAS version,Handbook of Chemistry and Physics, 67th Ed., 1986-87, inside cover.

The following are selected polymers used in the multilayer depositionsof the present invention: Charge/pH dependent Polymer Name PolymerAbbreviation Polymer Structure or independent Polyacrylic acid PAA

Anionic/pH dependent Polyallylamine hydrochloride PAH

Cationic/pH dependent Polyacrylamide PAAm

Neutral Polymethacrylic acid PMA

Anionic/pH dependent Polystyrene sulfonate SPS

Anionic/pH independent Polydiallyldimethyl-ammonium chloride PDAC

Cationic/pH independent Linear Poly(ethyleneimine) LPEI

Neutral Poly(ethyleneoxide) PEO

Neutral Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) PAMPS

Anionic/pH independent Poly(vinylpyrrolidone) PVP

Neutral Poly(vinylalcohol) PVA

Neutral Poly(ethylene glycol) PEG

Neutral Poly(aniline) PANI

Neutral Poly(styrene sulfonic acid-maleic acid, sodium salt) PSSM3:1

Anionic/pH dependent Poly(acryl-co-acrylamide acid, sodium salt)PAA-co-AAm

Anionic/pH dependent Poly(dimethylamine-co-epichlorohydrin) PDME

Cationic/pH independentMethods of the Invention

The present invention also relates to a method of forming a membrane,comprising sequentially depositing, under pH controlled conditions, aplurality of polymer layers on a surface; wherein each polymer layer isindependently selected from the group consisting of pH dependentcationic polyelectrolytes, pH independent cationic polyelectrolytes,neutral polymers, pH dependent anionic polyelectrolytes, and pHindependent anionic polyelectrolytes; wherein a polymer layer optionallycomprises at least one additional chemical entity selected from thegroup consisting of hydrogels, polyions, colloids, latexes, zeolites,platelets, proton sponges, organic molecules, organic salts, inorganicsalts, organic acids, inorganic acids, cationic dendrimers, anionicdendrimers, metals and carbon; and wherein said plurality of polymerlayers comprises a first polymer layer and second polymer layer; therebyforming a membrane.

The present invention also relates to the aforementioned method, whereinsaid membrane further comprises at least one additional chemical entityselected from the group consisting of hydrogels, polyions, colloids,latexes, zeolites, platelets, proton sponges, organic molecules, organicsalts, inorganic salts, organic acids, inorganic acids, cationicdendrimers, anionic dendrimers, metals and carbon.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid membrane is removed from said surface; thereby forming anion-exchange membrane or a LBL polyelectrolyte-carbon electrode.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid surface is selected from the group consisting of organichydrophilic porous filter membranes and inorganic hydrophilic porousfilter membranes; thereby forming an ion-exchange membrane.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid membrane is removed from said surface; thereby forming anion-exchange membrane or a LBL polyelectrolyte-carbon electrode.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid surface is selected from the group consisting of organichydrophilic porous filter membranes and inorganic hydrophilic porousfilter membranes; thereby forming an ion-exchange membrane.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid surface is organic, semi-metallic or metallic; and wherein said atleast one additional entity is carbon; thereby forming a LBLpolyelectrolyte-carbon electrode.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH dependent cationicpolyelectrolyte, a pH independent cationic polyelectrolyte or a neutralpolymer; and wherein said second polymer layer is a pH dependent anionicpolyelectrolyte or pH independent anionic polyelectrolyte; and whereinsaid at least one chemical entity is selected from the group consistingof metals and inorganic salts; and wherein said at least on additionalentity is carbon; thereby forming a LBL polyelectrolyte-carbonelectrode.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PDAC, PDME, PAAm, LPEI and PEO.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is selected from the group consistingof PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is selected from the group consistingof PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm.

The present invention also relates to the aforementioned methods,wherein said at least one chemical entity is selected from the groupconsisting of sulfonated latex, sodium chloride, potassium chloride,lithium chloride, sulfonic acid, nitric acid, hydrochloric acid,hydrobromic acid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP,polyphosphates, Nafions®, ethylene glycol and glycerol.

The present invention also relates to the aforementioned methods,wherein said at least on chemical entity is selected from the groupconsisting of sodium chloride, sulfonated latex, and Nafion 117.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI; and whereinsaid at least one chemical entity is selected from the group consistingof sulfonated latex, sodium chloride, potassium chloride, lithiumchloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromicacid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates,Nafions®, ethylene glycol and glycerol.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is selected from the group consistingof PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA; and whereinsaid at least one chemical entity is selected from the group consistingof sulfonated latex, sodium chloride, potassium chloride, lithiumchloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromicacid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates,Nafions®, ethylene glycol and glycerol.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI; and whereinsaid second polymer layer is selected from the group consisting of PAA,PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA; and wherein said atleast one chemical entity is selected from the group consisting ofsulfonated latex, sodium chloride, potassium chloride, lithium chloride,sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid,phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions®,ethylene glycol and glycerol.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PDAC, PDME, PAAm, LPEI and PEO; and wherein said at least on chemicalentity is selected from the group consisting of sodium chloride,sulfonated latex, and Nafion 117.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is selected from the group consistingof PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and wherein said at least onchemical entity is selected from the group consisting of sodiumchloride, sulfonated latex, and Nafion 117.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is selected from the group consistingof PDAC, PDME, PAAm, LPEI and PEO; and wherein said second polymer layeris selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1,PAA-co-PAAm; and wherein said at least on chemical entity is selectedfrom the group consisting of sodium chloride, sulfonated latex, andNafion 117.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is LPEI.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is a pH independent anionicpolyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PEO.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is a pH dependent polyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PAAm.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is a pH dependent polyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PDAC.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is a PH dependent polyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is a pH independent polyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PDME.

The present invention also relates to the aforementioned methods,wherein said second polymer is a pH independent polyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is PAMPS.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a neutral polymer.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a pH independent cationicpolyelectrolyte.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is SPS.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a neutral polymer.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is PSSM3:1.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a neutral polymer.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is PAA.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a neutral polymer.

The present invention also relates to the aforementioned methods,wherein said second polymer layer is PAA-co-PAAm.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is a neutral polymer.

The present invention also relates to the aforementioned methods,wherein said first polymer is a pH independent cationic polymer.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is LPEI; and the wherein the secondpolymer layer is SPS.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PSSM.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PEO; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PAAm; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PAAm; and wherein the second polymerlayer is PAA-coPAAm.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PDAC; and wherein the second polymerlayer is PAA-co-PAAm.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PDAC; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned methods,wherein the first polymer layer is PDME; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned methods,wherein the PAA is deposited at a pH of about 4.0.

The present invention also relates to the aforementioned methods,wherein the PAA is deposited at a pH between about 3.5 and about 4.5.

The present invention also relates to the aforementioned methods,wherein the PAA is deposited at a pH of about 2.0.

The present invention also relates to the aforementioned methods,wherein the PAA is deposited at a pH of about 2.5.

The present invention also relates to the aforementioned methods,wherein the PAA is deposited at a pH between about 1.5 and about 3.0.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH of about 2.0.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH between about 1.5 and about2.5.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH of about 5.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH between about 4.5 and about5.5.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH between about 5.5 and about6.5.

The present invention also relates to the aforementioned methods,wherein the PAA-co-PAAm is deposited at a pH between about 6.5 and about7.5.

The present invention also relates to the aforementioned methods,wherein said surface is selected from the group consisting of carboncloth, porous stainless steel, porous silicon, porous titanium alloysand gold.

The present invention also relates to the aforementioned methods,wherein said carbon is selected from the group consisting of carbonpowder, aqueous carbon paste, and Hispec3000 powder.

The present invention also relates to the aforementioned methods,wherein said metal is selected from the group consisting of Pd, Pt, Au,Ru, Zn, Cu, Ag and Al.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PDAC; and wherein said secondpolymer layer is PAMPS; and wherein said carbon is carbon powder; andwherein said metal is Pd.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is PDAC; and wherein said secondpolymer layer is PAMPS; and wherein said carbon is Hispec 3000; andwherein said metal is Pt.

The present invention also relates to the aforementioned methods,wherein said first polymer layer is LPEI; and wherein said secondpolymer layer is PAA deposited at a pH of about 4.0; and wherein saidcarbon is aqueous carbon paste; and wherein said metal is Pt.

The present invention also relates to a membrane, comprising a pluralityof polymer layers held together by electrostatic or hydrogen bondinginteractions, wherein said membrane has a total uniform thickness ofless than about 10 μm and a conductance of less than about 1.0×10⁻³ S/cmand this conductance does not degrade over time.

The present invention also relates to the aforementioned membrane,wherein said membrane has a total thickness of less than about 1 μm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a total thickness of less than about 0.75 μm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a total thickness of less than about 0.5 μm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a total thickness of less than about 0.25 μm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a total thickness of less than about 0.1 μm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 5.0×10⁻⁴S/cm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 2.0×10⁻⁴S/cm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 5.0×10⁻⁵S/cm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 2.0×10⁻⁵S/cm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 5.0×10⁻⁶S/cm.

The present invention also relates to the aforementioned membranes,wherein said membrane has a conductance of less than about 1.0×10⁻⁶S/cm.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is LPEI; and the wherein the secondpolymer layer is SPS.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PSSM.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is LPEI; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PEO; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PAAm; and wherein the second polymerlayer is PAA.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PAAm; and wherein the second polymerlayer is PAA-coPAAm.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PDAC; and wherein the second polymerlayer is PAA-co-PAAm.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PDAC; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned membranes,wherein the first polymer layer is PDME; and wherein the second polymerlayer is PAMPS.

The present invention also relates to the aforementioned membranes,wherein said membrane can be used as an ion-exchange membrane, a gasdiffusion membrane or an electrode.

The present invention also relates to the aforementioned membranes,wherein the said membrane can be used in a membrane-electrode assembly,a battery, a galvanic cell, an electrochemical cell, amicro-electrochemical cell, a catalytic converter, a solid-statehydrogen pump, an electrolyzer, or an electrochromic device.

Exemplification

The invention now being generally described, it will be more readilyunderstood by reference to the following examples, which are includedmerely for purposes of illustration of certain aspects and embodimentsof the present invention, and are not intended to limit the invention.

EXAMPLE 1 Membrane Preparation

All polyelectrolytes used to fabricate the LBL films were used asreceived. Poly(styrene sulfonic acid, sodium salt) [PSS; MW=70,000],Poly(diallyldimethyl ammonium chloride) [PDAC; MW=240,000], LinearPoly(ethyleneimine) [LPEI; MW=25,000], Poly(acrylic acid) [PAA;MW=90,000], Poly(styrenesulfonic acid-maleic acid, sodium salt)[PSSM3:1; MW=20,000], Poly(ethylene oxide) [PEO; MW=4,000,000],Poly(methylacrylic acid) [PMAA; MW=100,000], Poly(acrylic-co-acrylamideacid, sodium salt) [PAAcoAAm, MW=10,000,000 40% carboxy], all fromPolysciences Inc. Poly(2-acrylamido-2-methyl-1-propane sulfonic acid)[PAMPS; MW=2,000,000], Poly(dimethylamine-co-epichlorohydrin) [PDME],and Polyaniline(Emarlidine base) [PAN, MW=100,000], from Aldrich.Nafion117 as a 5% resin solution from Fluka. Sodium chloride fromMallinckrodt C.P., Sulfuric acid 98%, Nitric acid 96.6%, 30% hydrogenperoxide from Fischer.

Nucleopore® membranes (25mm diameter) were from SPI supplies. Nafion112®from DuPont, and ELAT® C/Pt electrodes (˜0.5mg platinum loading) fromE-Tek Inc.; Profilometer from Tencore Ins. Type P-10, Scanning electronmicroscope (SEM) JOEL 5910 and an environmental SEM (XL30 FEG-ESEM) fornon-conducting polymer samples, EG&G Princeton Applied Researchpotentiostat model 276, AC Impedance Solartron Inc. type SI 1260, plasmacleaner/sterilizer PDC-32G, Nicolet 550 SeriesII FTIR.

The track etched Nucleopore® membrane with 0.1 μm pores and 3×10⁸ poredensity was plasma etched for 90 seconds. A positively charged layer ofPDAC was first deposited to prepare the porous membrane for multilayerdeposition. For the entire PE couples used 40 bilayers were depositedusing an automated ZEISS DS50 dipper. Whether the LBL film was depositedon a Nucleopore or a Si-100 wafer the thickness of the compositemembrane or the LBL film was determined using a profilometer andcharacterized by FTIR. The Nucleopore membrane is fairly transparent toIR (around 75% transmission peak-to-peak on the interferogram window).

EXAMPLE 2 Fuel Cell Assembly

The “composite membranes” were soaked in 20.0 mM sulfuric acid solutionfor 20 minutes then placed on permanox slides ready to be folded betweenthe ELAT C/Pt electrodes. All the “composite membranes”, including theNafion112, were moderately hand pressed to the electrodes at roomtemperature. The untreated electrodes were twice soaked in 2.5%Nafion117 solution for 15 minutes and oven dried at 90° C. for 3minutes.

The Nafion112® was pretreated by heating at 80° C. with 2% H₂O₂ solutionfor 2 hours followed by rinsing with Milli-Q pure water. The Nafionmembrane was then soaked in a 0.5M sulfuric acid solution for two daysand then boiled in 20 mM sulfuric acid solution for 1 hour beforestoring in pH=1.7 H₂SO₄ solution. Slade, S.; Campbell, S. A.; Ralph, T.R.; Walsh, F. C. J. Electrochem. Soc. 2002, 149, A1556-A1564.

The MEA was fitted inside a fuel cell module that acts as bipolar platesbut with no serpentine channels that provide intimate fuel flow. Themodule was enclosed in a homemade two-chamber manifold where thepressures of air and hydrogen (BOC grade) were monitored and adjustedusing digital pressure gauges ACSI from McMaster-Carr Supply Company.The temperature and humidity in the H₂ chamber were monitored using aMASTECH MS6503 humidity-temperature meter. Pressures were never takenabove 10 psi and glass humidifiers in a water bath thermostat were used.

The power delivered by the cell had its cathode connected to the workingelectrode terminal while the anode was connected to both the referenceand the counter electrodes. Measurements were made using the EG&G 276software that dials a particular current in the galvanostat mode inorder to measure the corresponding potential as a function of time.Measurements were checked against resistive loads using an ammeter and avoltmeter.

EQUIVALENTS & INCORPORATION BY REFERENCE

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference in theirentirety for all purposes.

1. A method of forming a membrane, comprising sequentially depositing, under pH controlled conditions, a plurality of polymer layers on a surface; wherein each polymer layer is independently selected from the group consisting of pH dependent cationic polyelectrolytes, pH independent cationic polyelectrolytes, neutral polymers, pH dependent anionic polyelectrolytes, and pH independent anionic polyelectrolytes; wherein a polymer layer optionally comprises at least one additional chemical entity selected from the group consisting of hydrogels, polyions, colloids, latexes, zeolites, platelets, proton sponges, organic molecules, organic salts, inorganic salts, organic acids, inorganic acids, cationic dendrimers, anionic dendrimers, metals and carbon; and wherein said plurality of polymer layers comprises a first polymer layer and a second polymer layer; thereby forming a membrane.
 2. The method of claim 1, wherein said membrane further comprises at least one additional chemical entity selected from the group consisting of hydrogels, polyions, colloids, latexes, zeolites, platelets, proton sponges, organic molecules, organic salts, inorganic salts, organic acids, inorganic acids, cationic dendrimers, anionic dendrimers, metals and carbon.
 3. The method of claim 1, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said membrane is removed from said surface; thereby forming an ion-exchange membrane or a LBL polyelectrolyte-carbon electrode.
 4. The method of claim 1, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said surface is selected from the group consisting of organic hydrophilic porous filter membranes and inorganic hydrophilic porous filter membranes; thereby forming an ion-exchange membrane.
 5. The method of claim 2, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said membrane is removed from said surface; thereby forming an ion-exchange membrane or a LBL polyelectrolyte-carbon electrode.
 6. The method of claim 2, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said surface is selected from the group consisting of organic hydrophilic porous filter membranes and inorganic hydrophilic porous filter membranes; thereby forming an ion-exchange membrane.
 7. The method of claim 2, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said surface is organic, semi-metallic or metallic; and wherein said at least one additional entity is carbon; thereby forming a LBL polyelectrolyte-carbon electrode.
 8. The method of claim 2, wherein said first polymer layer is a pH dependent cationic polyelectrolyte, a pH independent cationic polyelectrolyte or a neutral polymer; and wherein said second polymer layer is a pH dependent anionic polyelectrolyte or pH independent anionic polyelectrolyte; and wherein said at least one chemical entity is selected from the group consisting of metals and inorganic salts; and wherein said at least on additional entity is carbon; thereby forming a LBL polyelectrolyte-carbon electrode.
 9. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI.
 10. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PDAC, PDME, PAAm, LPEI and PEO.
 11. The method of claim 1, wherein said second polymer layer is selected from the group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA.
 12. The method of claim 1, wherein said second polymer layer is selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm.
 13. The method of claim 1, wherein said at least one chemical entity is selected from the group consisting of sulfonated latex, sodium chloride, potassium chloride, lithium chloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions®, ethylene glycol and glycerol.
 14. The method of claim 1, wherein said at least on chemical entity is selected from the group consisting of sodium chloride, sulfonated latex, and Nafion
 117. 15. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI; and wherein said at least one chemical entity is selected from the group consisting of sulfonated latex, sodium chloride, potassium chloride, lithium chloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions®, ethylene glycol and glycerol.
 16. The method of claim 1, wherein said second polymer layer is selected from the group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA; and wherein said at least one chemical entity is selected from the group consisting of sulfonated latex, sodium chloride, potassium chloride, lithium chloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions®, ethylene glycol and glycerol.
 17. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG and PANI; and wherein said second polymer layer is selected from the group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and PAA-co-PAA; and wherein said at least one chemical entity is selected from the group consisting of sulfonated latex, sodium chloride, potassium chloride, lithium chloride, sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions®, ethylene glycol and glycerol.
 18. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PDAC, PDME, PAAm, LPEI and PEO; and wherein said at least on chemical entity is selected from the group consisting of sodium chloride, sulfonated latex, and Nafion
 117. 19. The method of claim 1, wherein said second polymer layer is selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and wherein said at least on chemical entity is selected from the group consisting of sodium chloride, sulfonated latex, and Nafion
 117. 20. The method of claim 1, wherein said first polymer layer is selected from the group consisting of PDAC, PDME, PAAm, LPEI and PEO; and wherein said second polymer layer is selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and wherein said at least on chemical entity is selected from the group consisting of sodium chloride, sulfonated latex, and Nafion
 117. 21. The method of claim 1, wherein said first polymer layer is LPEI.
 22. The method of claim 21, wherein said second polymer layer is a pH independent anionic polyelectrolyte.
 23. The method of claim 1, wherein said first polymer layer is PEO.
 24. The method of claim 23, wherein said second polymer layer is a pH dependent polyelectrolyte.
 25. The method of claim 1, wherein said first polymer layer is PAAm.
 26. The method of claim 25, wherein said second polymer layer is a pH dependent polyelectrolyte.
 27. The method of claim 1, wherein said first polymer layer is PDAC.
 28. The method of claim 27, wherein said second polymer layer is a PH dependent polyelectrolyte.
 29. The method of claim 27, wherein said second polymer layer is a pH independent polyelectrolyte.
 30. The method of claim 1, wherein said first polymer layer is PDME.
 31. The method of claim 30, wherein said second polymer is a pH independent polyelectrolyte.
 32. The method of claim 1, wherein said second polymer layer is PAMPS.
 33. The method of claim 32, wherein said first polymer layer is a neutral polymer.
 34. The method of claim 32, wherein said first polymer layer is a pH independent cationic polyelectrolyte.
 35. The method of claim 1, wherein said second polymer layer is SPS.
 36. The method of claim 35, wherein said first polymer layer is a neutral polymer.
 37. The method of claim 1, wherein said second polymer layer is PSSM3:1.
 38. The method of claim 37, wherein said first polymer layer is a neutral polymer.
 39. The method of claim 1, wherein said second polymer layer is PAA.
 40. The method of claim 39, wherein said first polymer layer is a neutral polymer.
 41. The method of claim 1, wherein said second polymer layer is PAA-co-PAAm.
 42. The method of claim 41, wherein said first polymer layer is a neutral polymer.
 43. The method of claim 41, wherein said first polymer is a pH independent cationic polymer.
 44. The method of claim 1, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PAMPS.
 45. The method of claim 1, wherein the first polymer layer is LPEI; and the wherein the second polymer layer is SPS.
 46. The method of claim 1, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PSSM.
 47. The method of claim 1, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PAA.
 48. The method of claim 1, wherein the first polymer layer is PEO; and wherein the second polymer layer is PAA.
 49. The method of claim 1, wherein the first polymer layer is PAAm; and wherein the second polymer layer is PAA.
 50. The method of claim 1, wherein the first polymer layer is PAAm; and wherein the second polymer layer is PAA-coPAAm.
 51. The method of claim 1, wherein the first polymer layer is PDAC; and wherein the second polymer layer is PAA-co-PAAm.
 52. The method of claim 1, wherein the first polymer layer is PDAC; and wherein the second polymer layer is PAMPS.
 53. The method of claim 1, wherein the first polymer layer is PDME; and wherein the second polymer layer is PAMPS.
 54. The method of claim 47, wherein the PAA is deposited at a pH of about 4.0.
 55. The method of claim 47, wherein the PAA is deposited at a pH between about 3.5 and about 4.5.
 56. The method of claim 48, wherein the PAA is deposited at a pH of about 2.0.
 57. The method of claim 48, wherein the PAA is deposited at a pH of about 2.5.
 58. The method of claim 48, wherein the PAA is deposited at a pH between about 1.5 and about 3.0.
 59. The method of claim 50, wherein the PAA-co-PAAm is deposited at a pH of about 2.0.
 60. The method of claim 50, wherein the PAA-co-PAAm is deposited at a pH between about 1.5 and about 2.5.
 61. The method of claim 51, wherein the PAA-co-PAAm is deposited at a pH of about
 5. 62. The method of claim 51, wherein the PAA-co-PAAm is deposited at a pH between about 4.5 and about 5.5.
 63. The method of claim 51, wherein the PAA-co-PAAm is deposited at a pH between about 5.5 and about 6.5.
 64. The method of claim 51, wherein the PAA-co-PAAm is deposited at a pH between about 6.5 and about 7.5.
 65. The method of claim 7, wherein said surface is selected from the group consisting of carbon cloth, porous stainless steel, porous silicon, porous titanium alloys and gold.
 66. The method of claim 7 or 8, wherein said carbon is selected from the group consisting of carbon powder, aqueous carbon paste, and Hispec3000 powder.
 67. The method of claim 8, wherein said metal is selected from the group consisting of Pd, Pt, Au, Ru, Zn, Cu, Ag and Al.
 68. The method of claim 8, wherein said first polymer layer is PDAC; and wherein said second polymer layer is PAMPS; and wherein said carbon is carbon powder; and wherein said metal is Pd.
 69. The method of claim 8, wherein said first polymer layer is PDAC; and wherein said second polymer layer is PAMPS; and wherein said carbon is Hispec 3000; and wherein said metal is Pt.
 70. The method of claim 8, wherein said first polymer layer is LPEI; and wherein said second polymer layer is PAA deposited at a pH of about 4.0; and wherein said carbon is aqueous carbon paste; and wherein said metal is Pt.
 71. A membrane, comprising a plurality of polymer layers held together by electrostatic or hydrogen bonding interactions deposited on a porous framework, wherein said membrane has a total uniform thickness of less than about 10 μm and a conductance of less than about 1.0×10⁻³ S/cm and this conductance does not degrade over time.
 72. The membrane of claim 71, wherein said membrane has a total thickness of less than about 1 μm.
 73. The membrane of claim 71, wherein said membrane has a total thickness of less than about 0.75 μm.
 74. The membrane of claim 71, wherein said membrane has a total thickness of less than about 0.5 μm.
 75. The membrane of claim 71, wherein said membrane has a total thickness of less than about 0.25 μm.
 76. The membrane of claim 71, wherein said membrane has a total thickness of less than about 0.1 μm.
 77. The membrane of claim 71, wherein said membrane has a conductance of less than about 5.0×10⁻⁴ S/cm.
 78. The membrane of claim 71, wherein said membrane has a conductance of less than about 2.0×10⁻⁴ S/cm.
 79. The membrane of claim 71, wherein said membrane has a conductance of less than about 5.0×10⁻⁵ S/cm.
 80. The membrane of claim 71, wherein said membrane has a conductance of less than about 2.0×10⁻⁵ S/cm.
 81. The membrane of claim 71, wherein said membrane has a conductance of less than about 5.0×10⁻⁶ S/cm.
 82. The membrane of claim 71, wherein said membrane has a conductance of less than about 1.0×10⁻⁶ S/cm.
 83. The membrane of claim 71, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PAMPS.
 84. The membrane of claim 71, wherein the first polymer layer is LPEI; and the wherein the second polymer layer is SPS.
 85. The membrane of claim 71, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PSSM.
 86. The membrane of claim 71, wherein the first polymer layer is LPEI; and wherein the second polymer layer is PAA.
 87. The membrane of claim 71, wherein the first polymer layer is PEO; and wherein the second polymer layer is PAA.
 88. The membrane of claim 71, wherein the first polymer layer is PAAm; and wherein the second polymer layer is PAA.
 89. The membrane of claim 71, wherein the first polymer layer is PAAm; and wherein the second polymer layer is PAA-coPAAm.
 90. The membrane of claim 71, wherein the first polymer layer is PDAC; and wherein the second polymer layer is PAA-co-PAAm.
 91. The membrane of claim 71, wherein the first polymer layer is PDAC; and wherein the second polymer layer is PAMPS.
 92. The membrane of claim 71, wherein the first polymer layer is PDME; and wherein the second polymer layer is PAMPS.
 93. The membrane of claim 71, wherein said membrane can be used as an ion-exchange membrane, a gas diffusion membrane or an electrode.
 94. The membrane of claim 71, wherein the said membrane can be used in a membrane-electrode assembly, a battery, a galvanic cell, an electrochemical cell, a micro-electrochemical cell, a catalytic converter, a solid-state hydrogen pump, an electrolyzer, or an electrochromic device. 